I arrive for the 5 p.m. Ganban-Yoga session on a Saturday evening, looking forward to my 30 minute Rock Bath, followed by a 60 minute Hot Yin Yoga class, on the rocks. Originating in Thailand, ganbanyoku spas are hugely popular in Japan. Hot rock bathing rocks!

Working out: my mortal enemy. I've never been one for strenuous activities. But I've gotten to a point where this is about being able to climb a flight of stairs without ending up winded. I've decided to not only catch up with the trendy sweaty regimens but to try them all out one by one and decide what works for me. First up on my fitness calendar was hot yoga.

Squeezing any physical activity into a hectic schedule is commendable, but does a toasty warm environment make a difference? Before attempting to main...