800m time trial during base, good idea?

Not sure why would you do it, but no, it won't.

As long as you warm up completely it should be no problem at all.

But do you literally mean "just doing mileage and tempos"? Different things work for different people, but I would recommend a Summer of Malmo (Winter of Malmo?) base period for most.

...Unless you're just doing marathon prep or something, in which case I guess you'd just be doing the 800 for fun.

And that's fine! Allegedly, people are in the sport for fun.

Achilles snapper events: jumps, sprints & throws. Ninety-nine percent on here when complaining about Achilles describe symptoms of micro tears. If you go from all easy mileage to 800m TT, you risk being taken down by Achilles Sniper. Achilles Sniper only gives out Ruptured Achilles.

Do it! Some times its fun!

No reason for a 800m TT during base IMO.

Assuming you are training for 800m, you base should include easy distance, tempos, and speed development (gym, plyos/drills, 5 x 80m hill accels, 5 x 60m, etc).

It may be a good idea to do a 5K (if less experienced, 3K/3200m/2 mile TT to check fitness & training paces.

As a speed test, maybe use 120m straightaway, avoid top speed / hard accel on the curve to keep it easy on that outside Achilles. 40m smooth accel, 80m flying TT.

I suggest, maybe do both TTs once a month, alternating every 2 weeks, during base. If TT's are fun for you

No, it won't hurt your base training if you don't taper, but it won't be a good 800.

Instead of a pure 800m time trial, maybe do an 800 at 1500m pace. That won't be easy either, but it would be better to do an even split controlled pace 800 with a little ragged form than something where you are falling apart in the last 100.

Go for it if you want. Just make sure you're feeling good, no niggles or tight spots, you don't want to waste a few weeks of training because of a time trial injury. Lydiard used to have his guys run time trials during base, I think he picked off distances like 300 or 500. Just to get some idea of speed and anaerobic power development. The off distances are often easier mentally because, like, what do you have to compare it to?

If you're so fragile during base training that going fast and hard for 2 minutes poses a serious threat to your body, then you're doing base training wrong.

What if I run 1600 pace for the first lap and try to hammer it second lap? Would that be better than running all out

what would be te point of doing that?

If you want to run an allout 800m, just do it.

Take a couple of days easy before and after and that's it, no problem.