Is it possible to build resistance to exercise?

One of the most important aspects of exercise is progression. If you are doing a gruling exercise routine your body will adapt to be able to complete it but if you keep doing the same workout improvement will slow.

I think you will have better results if you change your workout every 3 months instead of just trying to continuously increase intensity (there is a limit where you will find yourself on a plateau and unable to improve). Perhaps for 3 months really hit the weights to try to add some muscle; then the next 3 months work on running or swim speed with hard intervals; then for the next 3 months work on endurance by doing long steady exercise sessions; then do something else. Pros call this approach periodization.

Don’t worry about the water thing. Drink when you are thirsty. If you know you are going to do a workout where you will become dehydrated (like a long bike ride in 100F heat) you might pre-hydrate by drinking a pint or two before you start but other than that just drink when you are thirsty.