Intervals are a sharpening tool. They only work on an edge you've already built.
An interval session is any run that alternates hard efforts with recovery. Four minutes fast, two minutes jogging, five times over. That is it — the rest is detail.
What they actually do
Hard efforts push your body to improve the things easy running does not reach: how much oxygen you can use per minute, how well you clear the by-products of hard work, and — not least — how it feels to be uncomfortable and keep going.
That last one matters more than people admit. A race hurts. Intervals are where you practise hurting on purpose, in small doses, with a clock telling you when it ends.
When to start
Not in your first couple of months. Intervals put the highest forces through tissue that adapts slowest, so they belong on top of a base, not underneath one. If you have been running for six weeks, keep running easy and come back to this.
Start with strides
The gentlest entry, and the one our own plans use first: after an easy run, four to six efforts of about 20 seconds, quick but relaxed, with a minute of walking between. Not a sprint — think “fast and smooth”.
Strides cost almost nothing to recover from, teach your legs to turn over quickly, and make easy pace feel easier. They are in week one of the half marathon plan for exactly that reason.
Then proper intervals
When you want more, a first real session might be 5 × 3 minutes hard with 2 minutes jogging. Hard means you could not hold a conversation and could hold the effort for maybe ten minutes if forced.
Rules that keep it useful:
- Once a week. Twice is for people with years behind them.
- Warm up properly — at least ten minutes easy first, or the first rep is wasted and the risk is highest.
- Even is better than heroic. If the last rep is much slower than the first, you started too fast. A session where every rep matches is worth more than one that starts brilliantly and falls apart.
- Cool down afterwards.
The day after
Easy, or nothing. A hard session followed by another hard session is two mediocre ones, and the fitness lands in the gap between them, not during.