I keep an old set of tires around for when it's time to use the rollers. As for the extra energy, you're probably overheating unless you have a fan infront of you. This will make you feel tired a little quicker. Also, if you have a death grip on the bike from being afraid of rolling off, you need to relax because being tense isn't going to help anything. You'll get the hang of it and it'll get easier.
I have no theories as to why you were only able to keep a 12mph pace, that seems a bit odd.