Building Rome:
The Science Behind the build
Rome wasn’t built in a day.
It’s a phrase we hear all the time—definitely overused—but if we stop and think about what it really means to build something solid, something lasting; it becomes a powerful analogy. A city that has stood the test of time didn’t appear overnight. It was planned, layered, and strengthened piece by piece.
I often hear from runners trying to build up to something big—like training for a marathon—in a short window of time. And it's not the ambition that’s the problem (that’s the respectful part). It’s that this process simply doesn’t work with shortcuts.
Run fitness is like a city—constructed from hundreds of tiny bricks, laid one at a time with care and intention. Beneath the surface lies the hidden infrastructure: roads, aqueducts, and foundation stones. In the body, these show up as capillaries, mitochondria, and skeletal strength.
So why does it take so long to build durable run fitness? And how long is “so long,” anyway?
Because yes—I know. Runners love timelines and goals. :)
Let’s break down the beneath the surface systems that get built in the background when you train consistently and patiently:
1. Cardiovascular System: The Early Adaptor
When someone starts (or restarts) running, the cardiovascular system is usually the first to show measurable improvement. Within just a few weeks, your heart becomes more efficient—pumping more blood with each beat—and your body gets better at transporting and using oxygen. This means easier breathing, lower heart rates during workouts, and a general sense that running is becoming less effortful.
But they can also be deceiving. Just because your lungs and heart are adapting quickly doesn’t mean your bones, tendons, and connective tissue are ready to handle a big jump in volume or intensity.
That’s why it’s crucial to view early cardiovascular gains as the start of the fitness process—not the signal to leap ahead.
2. Mitochondria: The Power Plants of Endurance
When we talk about building “aerobic base,” we’re really talking about growing your mitochondria—those microscopic powerhouses in your muscle cells that produce energy. The more you have, the longer you can go before fatiguing. But again, mitochondrial biogenesis doesn’t happen overnight.
Research shows that in runners, mitochondria increase in both size and number within 8–12 weeks of consistent aerobic training. This isn’t just about improving endurance—it’s about building the cellular engine that keeps you going.
The recipe? Regular, submaximal aerobic work. Building volume. Easy runs. Time on feet. The kind of work that isn’t flashy—but is wildly effective.
3. Capillary Growth: Building the Roads
Capillaries are the tiny blood vessels that deliver oxygen and nutrients to your muscles. Think of them as the local roads and alleyways that connect your circulatory highways to every cell doing the work. The more you have, the more efficient and targeted your delivery system becomes.
When you consistently train, especially at moderate aerobic intensities, your body responds by not only increasing the number of capillaries but also their length and branching. In other words, your body literally starts building new roads—extending their reach deeper into the muscle fibers most responsible for running. These changes allow for better oxygen exchange, faster waste removal, and more efficient fueling at the cellular level.
This process doesn't happen overnight. It typically takes 12–24 weeks of consistent training to see meaningful structural adaptations. And like many endurance-related systems, capillary development responds best to volume and consistency, not intensity. Think time on feet, not time on the clock.
4. Muscular vs. Tendon Adaptation: The Lagging Team
While your cardiovascular system can adapt relatively quickly (within 6–8 weeks), your tendons and connective tissues often take twice as long—sometimes even longer. You’ll start seeing measurable muscle strength gains around 6–8 weeks, thanks to neural adaptations and early hypertrophy. But tendon remodeling, which includes increases in stiffness, tensile strength, and cross-sectional size, tends to lag behind.
Research shows that after 8+ weeks of targeted loading, tendons demonstrate moderate improvements in stiffness (SMD ≈ 0.7) and material strength (Young’s modulus, SMD ≈ 0.69), with smaller gains in size (cross-sectional area (CSA), SMD ≈ 0.24). In rehab settings—such as Achilles tendinopathy—12 weeks of heavy-loading protocols (~90% MVC) resulted in ~20% increase in stiffness and ~9% increase in CSA.
This mismatch—strength rising before tendons are adequately adapted—is a common cause of overuse injuries. That’s why it's essential to integrate weight-based strength training that includes safe, controlled movements over a sustained 12–24 week period. These techniques ramp up collagen production, structural remodeling, and tensile resilience in tendon tissue. In short: your muscles may feel ready, but your tendons need more time—and targeted stress—to catch up.
5. Bone Density: The Invisible Armor
Every time you run, you send a powerful signal to your bones: "Adapt to this stress. Get stronger."
But bones don’t respond as quickly as your heart or muscles. Instead, they follow a slower biological process called bone remodeling. Think of it as your body’s long-term construction crew: osteoclasts first remove old or damaged bone, and osteoblasts follow behind, building fresh new tissue. It’s an elegant system—but it takes time.
In the early stages of training, your bones may actually become slightly weaker before they get stronger. That’s because the breakdown phase happens faster than rebuilding. Only with consistent, progressive loading—meaning regular impact and stress over time—do osteoblasts start to outpace the breakdown, laying down denser, stronger bone.
This remodeling cycle unfolds over weeks and months. On average, it takes about 24 weeks to see meaningful improvements in bone strength and structure. That’s why big jumps in volume or intensity too early can be risky, especially for women, post-injury runners, or those coming back after time off.
You might feel fit on the outside—but your bones are still catching up on the inside.
5. The Nervous System and Efficiency
As you lay more bricks, your nervous system refines your stride. Efficiency improves. Stride economy develops. But again, this takes repetition—not just running more, but running well. Drills, strides, skill work—these are the finishing touches on the foundation you're laying.
Why 24 Weeks?
We’ve found that about 24 weeks—roughly 6 months—is the sweet spot for creating deep, systemic run fitness that holds up. That’s not a magic number, but it’s one supported by science and confirmed by experience. (You may remember from a previous blog: it takes 16 weeks to lose your run durability, and 24 weeks to rebuild it.)
This isn’t meant to discourage you. It’s the opposite. It’s permission to slow down. To stop chasing fitness like it’s a race, and start treating it like a structure you’re building for life.
So What Does This Mean for Your Training?
A quick note for athletes coming from other endurance sports (like swimming or cycling): you may feel aerobically fit, but that doesn’t always mean you're run-fit. Run-specific tissues like bones, tendons, and fascia need time and loading to catch up. Respecting that gap can save you from frustrating setbacks.
Consistency over intensity. Easy runs, run/walks, and frequent aerobic work are your greatest tools in the base phase.
Respect the lag. Just because you feel fit doesn’t mean your tissues are ready to handle full training. Ramp slowly.
Do the boring work. Plyometrics. Strength training. Drills. They're the scaffolding that makes sure your Rome doesn’t crumble.
Track your stress. Life stress, training stress—they all add up. Monitor your recovery, and prioritize sleep and down-regulation tools.
Give it time. There is no shortcut to bone remodeling, tendon adaptation, or mitochondrial expansion. But if you stay the course, you’ll be surprised what 24 weeks can build.
Building Rome wasn’t about one glorious day of construction. It was about showing up, brick by brick, day after day, season after season.
So, if you're training right now, know this: you're not behind. You’re building your city. And that takes time.
