The limiting factor is always cardio. My legs are strong enough to put out as much as 800 W for short periods of time, but, since my cardio system can't supply oxygen to the legs nearly as fast, if I try to go at 800 W, I get exhausted in 30 seconds. My cardio can supply oxygen for 200..220 W and I can go for an hour nonstop at that power level.

Comfortable cadence will increase with practice. Weight training (squats/leg presses) may help too.

You are probably not going to get to 300 W sustainable (cyclists often talk about max 1-hour sustainable power, or FTP). You can get to 200, you _might_ get to 250, 300 is very unlikely. You can exceed FTP for shorter periods - e.g. if your FTP is 200, you can go at 250 for 10 minutes with enough motivation.

It's much, much easier to cruise at 20 mph on a road bike than on an upright MTB.