Just Push Through?

Being on a mission trip for four weeks rather than two may be tricky. I normally push myself the whole time so I can be as effective as possible, but am I gonna hit a wall soon? We've only done one day of ministry and one day of leisure (though rafting the Nile isn't the most leisurely activity), but I've ended every day completely exhausted. I suppose that constantly drinking water and hanging out with God are the keys to making this work, but I haven't even begun working in the hospitals yet! I've been battling a cold the whole time, which is usual when I travel, but I don't think I can afford to be sick for too much of the trip. I was sick for the second half of my trip to the Philippines last year, and I just pushed through, but I dunno whether my body can take all of it.