As a runner with daughters also into athletics, I'm always having to work out pace, lap times, predicted finish times and so on. I thought it would be a nice challenge during lockdown to create a phone friendly mobile website with a bunch of utilities that could be useful. So far I've added:
The Split calculator and Stopwatch can work together, so you can transfer the suggested splits into the Stopwatch and it will then tell you if you are ahead or behind your required pace. Of course you can also use it as a plain stopwatch. The pace chart lets you pick different columns to compare, miles to km, km to 3k or 5k finish time, and so on. Pace to time estimate lets you put in a pace and pick a race distance, and it tells you what time you would finish if you ran the whole race at that pace.
You can find the beta of Runner's Toolbox here, it works best on a phone. I'll be adding new features as I think of them.