Stu9, I suspect that your coming on here and telling us you were thinking of quitting shows that, deep down, you really don't want to - you want/ need some re-assurance that you can do it, that things will get better - and some suggestions for routes/ rides. < amateur psychologist mode to OFF
>
Spokes maps are great
and you can always put the bike in the car (assuming you have one) and go somewhere quiet and, if need be, do some circuits of a park, out-and-back on a canal towpath or some other traffic-free space. Start small - a couple of miles, then give yourself a pat on the back and plan to do it again the next day. Build it up slowly, so you are enjoying it each time. As long as you are enjoying it, you'll keep doing it ... and the strength/ fitness/ distance will come - as many others have posted above !
Riding to a cafe/ pub gives incentive too
... and a chance to recover before the return trip.
20 miles is probably not a good idea at the moment, I agree; but if it is a
target to be able to ride for 20 miles by, say the end of May - well then you have a realistically achievable goal. After that, a 30-miler is next, then ..... Increased riding, with associated increase in riding skills, will give greater confidence, so you will feel more prepared to take to the quieter roads, then build up till you can take your rightful place on other roads.
Keep pedalling and Ride Safe !