My main objective is to ride for fitness and with my children for active family time. I also would like to run errands and pick up a few things once in a while. I ride on paved roads as well as dirt/gravel roads and some off road paths. I have 2 bikes. One is a single speed cruiser. I like the upright position of this bike as it is comfortable. My other bike is an 18 speed mountain bike. I am not particularly fond of the someone hunched position I feel like I am in when riding this bike. It has knobby tires which I have heard aren't good for the road but work for the off road places I go. I also like having the gears for climbing and descending hills but the single speed isn't bad for that either. I have a few questions:
1)Should I switch between the two or just pick one?
2)Should I get different tires for the mountain bike?
3)Is it advisable to modify the mountain bike so that the position I am in is more upright (like the cruiser) which I like or just give it time and adjust to this lower position?
4)Which of these two is the best for all around fitness?