If you're only riding solo, you don't need to get faster unless you want to ride faster just for the heck of it.
Since you just got back to riding, your old cycling strength is gone. You'll have to train to get it back. It won't be overnight. Would take months.
I know quite a few "roadies" who ride with hardtail MTB with wide knobby tires on paved roads and able to hold 20 mph for long periods.
Give it two months and then decide if you got back your old performance back or you want to get even faster with better equipment.