That's not such a low gear. By my calcs it comes to about 65 GI with a 700x23 tire, which is a totally reasonable road gear, especially for someone just getting back into the sport.
Riding a single gear means that effort and cadence are tied directly together. If your fitness won't let you ride faster than X mph, and your current gear has you turning the pedals over at 80 rpm at that speed, you have two options to increase cadence: gear down or pedal harder. IMHO, you're better off gearing down now to something like 42x19 or even 42x20, rather than grinding it out while you wait for your fitness to improve enough to let you spin 110 rpm in that gear for long periods.