MB,
There's no need to take a pony if you're in good (or even average) shape. Just take your time, bring snacks, and plenty of water, and remember that there's no need go at the swift pace your young guide might like to set. It's best to start in the morning when temperatures are cooler and get up to the temples at the top before the monk caretakers break for lunch. We went spent the morning hiking up and visiting the temples, and had lunch at the cafeteria on the way down--that left much of the afternoon free.
If you think altitude might be a problem, save the Takstang hike for the end of your trip and do shorter hikes to Tango/Cheri monastery (outside of Thimphu) first. For most people, the altitude of the places (ca 3000 meters, 10,000ft ) you visit on cultural tours are not a problem.
I'm a 60+ woman in moderately good shape and have hiked up to Takstang and similar temples (all on the tops of mountains) with no problems. Have a great trip!