How Fast Can You Go?

Pace Chart

How Fast Can You Go?

The goal of today’s post is to help you figure out how fast you should be trying to race. For example: You’ve run a 5k but you’re not sure what your pace should be for a 10k, half marathon or marathon. In the alternative, you have run a recent marathon and now you want to go back and try and set a personal best in the 5k.

Trying to guess a pace can be difficult. Running hard(er) workouts can make it easier, but there’s a good chance you still won’t know exactly how fast to run. Instead of going out too slow (and missing your potential) or going out too fast (and blowing up), check with Greg McMillan first!

Greg McMillan

Greg McMillan

Greg is an exercise scientist, runner and coach. He knows a lot more about running than I do, so I always check with him!

An Example

Last fall, I ran a 5k in early October and finished in 16:41. I was running a marathon several weeks later and wasn’t sure exactly what my pace should be. It had been years since I’d done a marathon. I knew I could probably run somewhere between 2:40 and 2:50, but other than that, I was only speculating. I went to the McMillan Running Calculator and punched in my 5k time. It indicated that I should be able to run around 2:42:34 – and sure enough, I ended up finishing in 2:42:55, only 21 seconds off!

Another Example

Here’s another example. Suppose you are training for a marathon and you recently completed a 10k in exactly 50 minutes. If you use the McMillan Running Calculator, it will tell you that you can run a marathon in about 3:54:35. It also tells you that 3:54:35 is 8:57/mile. And there you have it – you have a realistic goal pace for your marathon.

An Additional Benefit

The calculator can help you out with pacing, but it can also help point out weaknesses in your training. For example, suppose you ran a recent half marathon in 1:30. The calculator says you should be able to run a 5k in about 19:26. If there’s no way you could do that, perhaps you aren’t doing enough speedwork. In the alternative, if you can run 19:26 for a 5k but you can’t run 1:30 for a half marathon, perhaps you are not doing enough long runs.

Does This Always Work?

Unfortunately, no. I have found several instances where this tool is not as reliable:

1) the elevation profiles are not comparable – for example, if I punch in a 5k time that was run on a flat course, I most likely will not be able to match the implied marathon time on a hilly course

2) the terrain is different – for example, if I punch in a time that was run on a road course, I most likely will not be able to match the implied time on a trail course

3) the inserted time was not a good result – for example, suppose you have a bad day and run a lousy 10k time. Do not use that time in the McMillan calculator – the resulting time would not accurately reflect your potential


The McMillan Running Calculator is not foolproof. However, I think it can be a very useful tool, especially if you are relatively new to running. Over the years, I’ve found it to be pretty reliable and it has most likely prevented blow-ups on more than one occasion.

Give it a try and let me know what you think!

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6 Responses to How Fast Can You Go?

  1. emmelineruns says:

    My favorite is using his calculator to help me figure out paces for my training. When I first started intervals years ago, I thought you were supposed to run them all out…no wonder I hated interval training. I like using his suggested paces to be sure I’m not turning a tempo run into a race or a long run into a steady which I can definitely be guilty of!

  2. I use a site (runningforfitness I think) with a great calculator. They do say that the accuracy is dependent on us training correctly.. So like you said if we are not doing enough speed work etc then the results wont be accurate.

  3. I am definitely a McMillan disciple, I love his training plans and the logic behind them. He himself bases a lot of his training on Arthur Lydiard’s philosophy of course, which suits me perfectly well as a high-mileage runner. I often use the calculator to work out my training paces and get an idea of race times, and I find that it’s surprisingly accurate. The other important thing to remember is that it obviously assumes that you have trained for the distance (but that should hopefully be a no-brainer!). P.S.: Man, you are fast! 😉

    • Jason says:

      Thanks! And it sounds like we’re on the same page! By the way, good point – I probably should have included that more clearly in the post – train, train, train!!

  4. Pingback: VDOT Running Calculator | Must Love Jogs

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