Wow! Good luck! You have a challenge...
I had the exact opposite problem, my "stoker wench" (call her that cause she's not always nice to me) is a whirlwind of excitement and a power house of energy, i don't know where she gets it from, those slender legs.

So anyway, one time we were hammering up a very steep set of hills on Vancouver Island and i was just about ready to die, we were full-loaded for god's sakes. she yells "don't give up on me now!" i knew something was going to give, i lost my voice the next day and couldn't speak for a week. she pushed me too far.

five years later i got her back when we ascended mt. baker in washington, she didn't want to do it but i persuaded her with my culinary skills. we pushed very hard that day for 8hrs. she just about got hyperthermia and almost passed-out. i made her climb all 4100ft. in the sun with water rations. ha!
yes, we do have fun on that tandem.

later.

mike.