Do you know what kind of mushroom you will be taking?
You don't need to work out. I think the legend is that to overdose on magic mushrooms you have to eat your bodyweight's worth or something.
The fact is that it's not likely to happen. Confirm they are propper shrooms, start with 0.5g to 1g dried and take more if you feel comfortable, and make sure you don't take more than 1.5g-2g dried the first time.
Importantly, if I were you, I'd read trip reports and shroom info (try www.magic-mushrooms.net, www.shroomery.org, www.erowid.org) before taking them until I was confident I was not going to die.
Try this article for example:
http://www.shroomery.org/index/par/12196
There's also a 'dosage calculator' here, for what it's worth:
http://www.shroomery.org/index/par/12195