26" low speed

I'm age 67, 160 lb, ride a 27 mile route to the summer camp on a kid's mountain bike 2 to 4 times a week. I'm 68" with 28" inseam so the 18" frame length of a kid's bike is no problem. I need a way back to town if I strain a muscle or tendon out there or get an infection cutting my wattage in...