I agree with the above, though I will say that on cellular, I get slightly worse results, though I am pretty sure that's because I'm in a marginal area, and the watch is likely looking for a signal a lot.
If you get a GPS only version or are ok with turning off the cellular, you should be fine. If, during your long runs in building up to the half, you see the battery isn't cutting it, you can do things like turning off the screen auto-wake to extend battery life even more.