In the F3 launcher, when I activate mods in the data files menu, why do they uncheck themselves when I close the data files menu?

Also is there a console command for increasing walk speed? I know there's one for changing running speed.

Im not sure why some peoples launcher (data section) will uncheck ... and others will not. And its not certain it unchecks untill you open it again. Hence don't open it again , and just launch with the Fallout3.exe

But the tried and true method is to use a mod manger. Which is a 3rd party program to do what the data section of the launcher does , plus more.

On the other thing you asked ... here is a page for it ... http://www.neoseeker.com/forums/43594/t1864892-tips-to-increase-movement-run-speed/

But you might want to check into mods that specifically do that.

I just tested the player.setav speedmult And it worked fine ... Set at 200 , I was able to back up crouching faster than a mole rat could run me down. Just remember all the other things relating to player speed will still apply proportionally.