How long does it take to build up lactic acid tolerance?

Does" lactic acid" tolerance even happen? Seems more like the body has different methods than developing tolerance.

Do you have higher levels as you get fitter?

55 is possible, but getting it by the end of the month is not probable.

Lactic tolerance develops in a couple of weeks, but that's not what's limiting you. You need more speed endurance. Try doing 2x(4x200) at fastest possible average speed with 30 seconds between reps and ten minutes between sets. Another day, try 300s with full recovery. Do uphill sprints after your main workouts (not too steep, start at 8 seconds per rep and work up to 15).

Don't ever flail. If your form starts to completely fall apart, then pull the plug and note how far you got. Then just wrap up with some shorter stuff.

Oh, and it's impossible to predict how quickly you'll respond to training. There is as much variation in the degree to which people respond as there is in their "raw" speed.

About 6-8 weeks to fully build up your tolerance, but you can develop most of your tolerance in the first 4 weeks. It's going to be brutal, though:

All out 500, 400, 300. Full recovery between each.

300 at 95%, rest 45 seconds, then all out 100. Full recovery, repeat 3 times.

3X400 at 98% with 4 minutes rest. If you're not puking by the end, you didn't run them hard enough.

Go on 2-3 mile "long runs" on your easy days. Unless you're running sub 50, there's a substantial aerobic component to the 400. There's usually 1-2 distance guys who are fast enough to make our 4x400 team despite not doing much speedwork.

Couch to 0.4K wrote:

After adding a quarter second or so to convert hand times to FAT, I'd estimate that my FAT times would be low 12s for the 100.

What would be a realistic 400m goal time that I can achieve by the end of the month?

By the end of the month? High 55s, low 56.

By the end of the year? Sub 54. Multiply your 100 time by 2 to get your 200 time, and multiply your 200 time by 2.2 to get your 400 time. 12.3 100 = 24.6 200 = 54.12 400. Your 100 time would probably improve by a tenth or so, so make that sub 54.