Jay a couple things to think about:
It is good you are getting back into riding. I was in a similar position where I got back in to riding after taking a break and thought how hard can 13 miles be. Well the short of it was harder than I expected.
So with that said you probably are using muscles you have not in a while and it will take some time to build up endurance.
A simple thing to focus on when starting is first be able to ride for a certain period of time on flat terrain that lets you pedal for 45 minutes straight. If you have a cadence meter you can focus on keeping your rpm between 75-90.
The difficult part is with a fixie or single speed bike, you as the rider have to compensate in effort for the terrain. While 48X19 is not too tough to push for a road bike cyclist, starting out on certain hilly terrain that can be less than ideal (e.g most road bikes come with 50X(34-11) ).
You have two options. One is you could train a bit on a stationary cycle/turbo trainer to build up endurance and strength. The other is you could get another bike that has gears in the back that will allow you to ride outside and not feel like you have to work as hard on hills.
If I was to go the route of a road bike with gears, I would go with something that has a 105 groupset at least and this way once you build up endurance and strength you have two options of great bikes to ride vs. one of them feeling like it is just the other bike.
I know that may not be the answer you want to hear, but usually people on fixie that are riding them far have some time on other bikes that they have built up their endurance on or they are not worried about how fast they get to their destination because they know they may have to go slower to compensate for the lack of gears.