A first marathon at 40-plus is one of the most satisfying things you can do, and it's well within your reach. But make no mistake, it demands respect. Sixteen weeks is a sensible runway if you can already run comfortably for around 45 minutes; much less than that and you're better off building a base first. No shame in it.
The honest shape of it looks like this.
The four phases
A good build moves through four stages, and (unlike the deep-dish pizza I once optimistically ordered in Chicago) you can't just skip to the good bit.
Base. A few weeks of mostly easy running, nudging your weekly volume up gently and getting your body used to being on its feet. Unglamorous, a little boring, and the foundation everything else stands on.
Build. The bulk of the plan. Your long run grows, and you add a weekly dose of something stronger, threshold work to lift the pace you can sustain. The rest stays easy, and that lopsided-looking split (lots of easy, a little hard) is exactly what the research keeps finding works best (Seiler & Kjerland, 2006).
Peak. The hardest few weeks, and you'll feel every one of them. Your longest runs and most marathon-specific sessions, rehearsing race pace until it feels almost automatic.
Taper. The final two to three weeks, where you deliberately cut the volume right back so you arrive fresh. It feels deeply wrong. You'll get twitchy, convinced you're leaking fitness by the hour. Trust it. The work is already in the bank; the taper just lets it float to the surface.
The long run is the engine
It's the most important session in the plan, and the most abused. Done properly, it climbs gradually from where you are now toward race distance, a kilometre or two at a time, with the odd step back, never a great heroic leap. And you don't need to run the full 42km in training; getting comfortably into the low-to-mid thirties is plenty for a first-timer.
The mistakes that end most first attempts (I've made both, so you don't have to)
The first is the long-run spike. Someone misses a couple of weekends, panics, and jumps from 18km straight to 30km to "catch up". That single oversized run is exactly the pattern most strongly linked to injury, far more than your total weekly mileage (Garmin-RUNSAFE, BJSM 2025). Miss a week? Pick up roughly where you left off. You can't win it back, and trying is how the wheels come off.
The second is going out too fast on the day. Rule 1 of marathon running is don't go off too fast. Everyone knows it. I knew it. And in Chicago I went off too fast anyway, banked a half-marathon PB somewhere in the first 16 miles, and paid for every second of it over the last ten. Learn from my idiocy: hold something back early, spend it late.
Why recovery weeks matter more now
You get fitter when you recover, not when you run: the run is the stimulus, the adaptation comes while you rest (Bishop et al., 2008). And after 40 that's more true, not less. A good plan has an easier week roughly every fourth: lower volume, a chance for all that work to land. Skipping them to "stay on track" is precisely how people arrive at the start line tired instead of sharp.
What a week tends to look like
- 3–5 runs. Whatever you can sustain beats an ideal week you can't.
- One long run. Easy effort, building gradually.
- One quality session. Usually threshold, later some at goal-marathon pace.
- The rest easy. Conversational, no heroics.
- Rest and cross-training as part of the plan, not something to feel guilty about.
Do that consistently for sixteen weeks and you'll line up ready. Not because you're 40-something, but because you trained like someone who fully intends to finish strong and in one piece. The marathon owes you nothing and promises you nothing. But put the work in, respect the distance, and it has a habit of handing you a day you'll never forget.