Just run until your eyeballs pop then stop!
Ran without a GPS in the 70s and still managed to run quite quickly when it came to race day. Our intervals were guessed but that doesnt matter, comparison with the same distance (under or over) is all that matters. I had a feeling that I wasnt running under 2 mins for my 800M intervals but so what 🙂
Find a loop that looks appropriate and run it regularly, time yourself sometimes and occasional try to run it faster so that it really hurts, mission accomplished. The only distance that counts is the one when you are wearing a number. Everything else is prep.
as an aside I remember turning up to run a (short) Monday night 10 with my clubmates only to find it wasnt short after all (well about 100m) we had been running it regularly in about 55 mins thinking it was about 9ish!