What Time Trial should I do?

Just run a full 1600m and be done with it. You'll know for sure what shape you're in since you ran the full distance and possibly a few sec faster factoring in if competition was present.

The rule of time trials:

Decide on your distance (in your case, a mile/1600).

Start off.

Decide 120 meters in that you're doing 75 percent of your intended distance.

Decide one minute later that you're doing half your intended distance.

Decide 30 second later that you're finished.

You've done a 600 meter TT. Nice work!

Do the mile. Report back ??

In some ways it makes sense to do the full 1600, since that will give you a more precise indicator of fitness.

Some thoughts: if you plan to run the 800 as well as the 16 you might want to do an 800 TT, because you can probably estimate your long distance (5kish) fitness from base, and use the 800 TT to interpolate (is that the right word?) your 1600 time.

I also find it much easier to run close to my fitness in a TT without competition when the distance is short (for me like .5-1 seconds slower over 400, 3-5 seconds slower over 800, 10-15 seconds slower over 1mi). I think I am unusually bad at solo time trials though, so take this with a grain of salt. (I just can't push myself to that race edge or stay out of my own head without competition).

Finally, I think there is some mental advantage to using an off distance (e.g. 1200), because you can approach the race fresh and be more focused on effort than pace, which I've always found to produce faster results.

Those are my thoughts. As a HS senior you should be smart enough to decide what you want to do, based on what you hope for this TT to accomplish.

Finally, once you finish the TT, I would give yourself 10 or 15 min and then try to make it into a workout (do some 400s or 200s).

Do a 3200 or even 5000 time trial. You want to assess your current aerobic strength. It will be tough but it will make you a better miler in the long-term. Expect to be 10-15 sec slower/mile than a race effort. Set you watch to beep for a reasonable anticipated pace and stick to it, then try to have a strong last lap.

YMMV wrote:

Do a 3200 or even 5000 time trial. You want to assess your current aerobic strength. It will be tough but it will make you a better miler in the long-term. Expect to be 10-15 sec slower/mile than a race effort. Set you watch to beep for a reasonable anticipated pace and stick to it, then try to have a strong last lap.

Depends on the purpose of the TT, but I think this is probably the right idea for OP's purposes at this point in the year. This will give you a good idea of your aerobic fitness for setting workout paces.

I do find that my race pace is usually about the same as my solo TT pace for 3/4 the race distance, so I'd use a 1200m TT to gauge my mile fitness or a 600m TT to gauge my 800m fitness. I'd use this more as an late season fitness check as I'm trying to peak rather than an early season TT. When I do this type of TT, I'd do the 3/4 distance TT, then short rest, then run 1/4 of the race distance hard. So 600m, then 200m hard; or 1200m, then either 400m or 2X200m hard. Basically doing the full race distance broken up into a TT then a hard repeat or two.