First, you don't need a gym membership to be in shape!
You might want to spend some cash on resistance bands! Many of them even come with DVDs.
Libraries also have exercise DVDs to loan for a cheap fee. I'm sure you can find prenatal yoga DVDs there too.
Finally, if you have cable TV, you can find "on demand" exercise programs.
I'm a professional fitness instructor & certified as a personal trainer (although fitness isn't what I do full time.) During my pregnancy, I continued to teach yoga, step aerobics & cardio kickboxing 2X per week, do boxing class that includes hitting & kicking heavy bags 1X per week, and lift weights heavy
(no playing around with pink plastic dumbells!) around 2X per week.
I was in excellent shape & I do feel this contributed to my relatively quick & easy birth!
1st stage was under 5 hours from the first, "Oh, yeah, that was a ctrx!" to the urge to push. Pushing DS out took about 45 min. Of course, I also did optimal fetal positioning exercises too.
And this isn't just my personal observation based on my own experience. Dr. Clapp conducted extensive research on it & wrote a book, "Exercising Through Your Pregnancy." Sure enough, the exercising mamas had faster labors, less use of epidural, less instrumental deliveries, and fewer CS! They even found benefits for the babies years down the road! I found the book to be a fascinating read, but then again, I find physiology interesting.
To qualify as "exercising" mamas, they had to exercise a minimum of 3X per week, for at least 20 min, at "moderate to hard intensity." So, unfortunately, walking doesn't count unless you're doing speedy power-walking, or going up hills, stairs, etc.
Finally, to answer your question of 'how taxing is it really?" Well, it varies so much. Baby's position helps - a baby that is, for example, occiput posterior ("sunny side up" - facing your belly) will result in labor that most say is more painful, and is also longer because that alignment is not as efficient at dilating the cervix. Both 1st and 2nd stages can be longer & tougher in this case.
For me, the first hour of labor was just cramps. Even the rest was sometimes painful, but manageable. Pushing was actually kinda fun & even though I'd barely eaten all day (just had no appetite), I don't remember feeling tired. As a matter of fact, DS was born at 6:50 PM and I was so HIGH on endorphins that I had trouble getting to sleep that night. I kept reliving the birth in my mind.
I think your best bests for a good birth experience are to choose a good HCP & good provider, take a good class (Bradley or Hypnobabies) & practice your method, eat a healthy diet & keep a positive attitude. Personally, I don't think you need to be in marathon-runner shape to have a good birth & have enough energy for a drug-free birth, so I wouldn't stress.