We booked our cave tubing with Reggie at
www.regtours.com and saved about $25 or $30 per person (can't remember exact amount, just know it was a good bit for the four of us in our family). Reggie was GREAT and very professional. I've previously written detailed descriptions of the excursions on this board and
www.cruisecritic.com (go to some of the previous pages for entries about Belize and cave tubing).
The walk is about 30 minutes along a relatively level path through some trees. You will be caring an inner tube, upon which you'll float down the river when you get to the caves. On our excursion with Reggie, we had people from 9 to early 60's and it is not a strenuous walk. You won't want to take much with you, because whatever you do take WILL GET WET. After your tubing, you'll walk back, carrying your inner tube, and be able to towel off and change into dry clothes (which hopefully you left in Reggie's van), as there is a small bathroom/changing area.
Others on these boards have also had very good experiences with XTREME tubing or adventures. However, I can't speak for them. Just know Reggie provided us with a quality, safe and professional excursion.
Steve