So if you enjoy mountain climbing you'll postpone it until it is safer or after the Singularity when you can have much more safe mountain climbing.

That presumes no time discounting.

Time discounting is neither rational nor irrational. It's part of the way one's utility function is defined, and judgements of instrumental rationality can only be made by reference to a utility function. So there's not necessarily any conflict between expected utility maximization and having fun now: indeed, one could even have a utility function that only cared about things that happened during the next five seconds, and attached zero utility to everything afterwards. I'm obviously not suggesting that anyone should try to start thinking like that, but I do suggest introducing a little more discounting into your utility measurements.

That's even without taking into account the advice about needing rest that other people have brought up, and which I agree with completely. I tried going by the "denial of pleasures" route before, and the result was a burnout which began around three years ago and which is still hampering my productivity. If you don't allow yourself to have fun, you will crash and burn sooner or later.