I'm not trying to be 'heavy' about this conversation.....but a few thoughts.
Anything that makes you active, get outdoors, exercise and improves your overall health is fantastic! Congrats on the motivation.
Riding a bicycle (road, hybrid, mountain, recumbent etc...) is a strenuous, active and challenging activity. As with any challenge....there are some risks.
Riding a tricycle (not recumbent 3 wheel, a real ADULT trike) provides some exercise, gets you off the couch, and could assist with mobility. There are some risks here too.
I would suggest you confer with your healthcare provider in person, on the phone or via email before undertaking such a challenge. You may have a free 'nurse-line' or health advisory contact to avoid spending any money for this advice. I only say this because your tone and your words tell me that you have concerns and you may have private reasons for those concerns. You shouldn't ignore your conscience. IMO
I would not suggest you purchase an adult trike with the intent to ride it on public roads or to the store. They are large, awkward, and don't mix with traffic, people or sidewalks. They especially don't mix well with fast-paced or busy bike lanes. They are usually quite heavy and can generate a lot of physical workout or stress on anything but flat pavement. For those that have mobility issues and/or private low-traffic areas to relax and enjoy a self-paced ride.....I think they are fantastic. Just like a golf cart, but with some exercise built in. But, make no mistake--you can injure yourself or threaten your health on anything with 2, 3 or 4 wheels. I have some experience with this....my Mom etc...
- New bikes are like new cars: drive them off the lot, they lose 30% of the value. It's your choice of course.
- Walmart sells awesome toilet paper (when they have it). They sell CRAP bikes. Pun intended.
- Buy used. Buy a decent bike made in the last 10 years that's made from decent materials. Have it inspected before you buy. Your LBS can help.
- If you buy a bike, like a car, you will need a mechanic some day. Buy new or used from your LBS (local, bike store). Or, just use the services of your LBS (inspection, tune-up etc.)
- Rent first. You can't possibly test-ride and select a good bike if you haven't ridden in years. Take it slow.
- Rental or test ride?: NO busy city streets. No curbs, cars, traffic, sewer grates, potholes. No large groups of people watching you relive your childhood.
- Find a spotter to assist you and keep you safe for the first few trial rides. As we get older....safety first.
- Never ride--even a short 'test'-- without appropriate safety gear. No helmet? Buy or rent one before you ride. No excuses.
- The most dangerous parts?: starting, stopping, getting on, getting off. Next comes traffic, curbs, grates, potholes. That's why you begin in a quiet spot.
After you see if riding a bike is safe and something you're actually motivated and capable of, you'll feel much better about spending the money and taking the time to find a bike that's a good fit for you. Or, you may find that biking is not physically appropriate for you or that you lack a safe environment to ride in.
In closing, I wish you luck. Please don't think that having a 3rd wheel or acid-paste will keep you safe or make riding 'much easier.'
Just like timeshares.....
.buying resale after doing your research will make you a happier Tugger!

My baby: (file photo)
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