birdmove

So, I'm a 67 year old cancer survivor with a bad right knee. My bike is a cheapo Thruster Fixie by Kent I believe. It's a flat baf road bike with C700 tires. Weight, as ridden (with a seat post rack, water bottle bracket on the handlebar, and an extendable baton on the rack for emergency dog protection) is about 31 pounds. I ride a 2.5 mile course every day, with a number of hills. The last half mild is so has 6 hills, the steepest I get down to very slow speeds, but I always clear all of them. Yesterday, I fixed a flat on my 39 year old daughter's Huffy Cranbrook cruiser bike, also single speed with coaster brakes. My bike has rim brakes all around. Fixed the flat, wired up the tires and lined the chain on the cruiser. I had done my ride in the am, and decided I'd read my run on the cruise to compare. The gearing on my bike is 2.66 gear ratio, don't know on the cruiser bike. Felt roughly the same. Weighed the cruiser in at about 37:2 pounds as ridden, so about 6.2 pounds heavier. Yesterday I set a time record for me on my bike of 11.01. When I began this about 3 weeks ago, I was at about 12:40 or so. Do, the 11:01 time run, everything went smoothly. Didn't have to stop or slow for cars, etc. So, the run with the heavier cruiser bike came in at a whopping 12:45. Due to the weight, and possibly s difference in gearing, rolling resistance etc, etc, and maybe I wasn't as strong after doing my morning run 3-4 hours earlier, though I did make all the hills, I was just generally slower. I know I can Gey my Thruster going faster in general. Is the cruiser rideable? Sure. Would I want to go on much longer rides with more hills? Not at this time. I should point out that, the knee is an obvious hindrance, but the two years of cancer treatment ending in October of 2018, really knocked my strength and stamina for a loop. I am working to gain back what I can. The daily ride, a daily walk, followed by some weight work right after the walk, and it seems to be helping some as shown by my improved times on my ride when riding MY bike anyway.
The Huffy has a steel frame, but so does my Thruster. I am considering getting s bike with some gears as I would like to do some longer rides for practical errands. Those rides would start at 15 miles or so, and if I can build myself back up to riding to Hilo and back, with a bit of running around there, that would be around 30 miles round trip. I had been doing rides like those on the Thruster about 6-7 years ago before blowing my knee up, but don't know if I'll be able to do it on a single speed bike again. I have read about bikes like my Thruster, with a weight of 21 pounds or so, and that would help. I won't buy a cruiser, I don't think. Think I'll stick with something lighter with skinny tires. I,'m looking at the Priority Classic, an internal hub 3 speed, 25 pound bike with belt dive, and one built to fight rust in general. It's about $700 shipped to me in Hawaii though. My current bike is super easy to work on, which I love.