Cycling on and off road is great fun. Before back troubles ended my cycling I used to get in 2.5 to 3.5k miles a year. Several century rides, (100 miles) and a 137 Aiken to Charleston, SC ride. I like it because you are going somewhere not just exercising in place.
Your neck muscles will strengthen for the road bike, and the idea is to keep most of your weight on the peddles not the seat or the bars.
Seat height and fore/aft adjustments are critical to your knee health. Spinning faster in a lower gear is also much easier on joints than grinding away on a higher gear.
I know you know about hydration, wearing gloves and a cycling helmet.