It won't be your aerobic fitness that limits you, more likely to be aches and pains from different muscles, sitting on a saddle, etc.
Often recommended for beginners is to work out a kind of figure 8 route, with home in the middle - so you do a loop (maybe 5 miles) and if it's going well you do the other loop. If not, you're close to home.
If bits do hurt, don't push on too far through the pain - better to fiddle with the bike fit until it IS comfy.
But enjoy!!