I fear I am going to need new tires soon (20" front, 26" back). So thought I would ask a few questions and get your opions. I am heavy (300 Lbs) so I want to get something that will hold up to punctures pretty good. Also I have to ride on dirt/grass (mud if wet) enroute to the paved trails, so I thought that may rule out no tread (slicks). Is it better to get some of those tire liners and thinner tires, or heavy duty tires to start with? What about airless tires? I am just a recreational rider (for exercise and just the joy of riding). Is there a certain pressure I should be riding? My current tires I run 40 PSI front and 60 PSI back (40 is the MAX for the front and 65 is the MAX for the rear). Also the trails I ride often have debris (sticks, pine cones, sweet gum balls, etc).