I'm 74 with no physical limitations affecting me in urban areas. Frankly, I worry more about the wheels on my suitcase than my body. I try not to pull the bag farther than a mile, but as a budget traveler who doesn't book hotels very far in advance, I have often needed to do so.
The first thing I want to do when I arrive in a new city is get rid of my suitcase. I look for a hotel (other options often not allowing an early drop-off of the bag) located close to the train station but in the direction of the historic center. After a couple of unfortunate choices that had me staying in food deserts, I now verify I'll be in an area with a bunch of restaurant options not too far away. Alas, hotels in areas I prefer often cost more than those farther from the train station.
I'm aware my approach is somewhat illogical in cities where I'll have rather lengthy stays (which is many of them). I'd do better to get a taxi from the train station--to save the wheels on my bag--and head to a less expensive hotel that would be closer to the sights. That would save money, overall. But I don't like the way some taxi drivers queued up at train stations like to rip visitors off.
I've been to Plovdiv but didn't arrive or depart early in the morning. I'd expect to find a taxi available at the station when the train arrives. That's generally the case in Europe in cities of any size, and Plovdiv has a population of over 300,000.
I must warn you that there are some really, really gnarly cobblestones in the historic center. They aren't exceptionally large like those in Pompeii, but they are very, very bumpy--so much so that I asked a local woman how they managed not to fall when wearing heels. She said, "Oh, we fall sometimes." You may be able to get an idea from some of these photos, though in my memory the cobbles were worse: https://www.bing.com/images/search?q=photos+plovdiv+cobbles&form=IACFSM&first=1
