I started riding again at 60 and am now 66, my first bike was a hybrid with flat bars. Knowing that I had not ridden a bike in decades I resisted the temptation to start changing out the seat and bars even though I was horribly uncomfortable at times, at one point I was convinced that the flat bars were just not for me I pretty much hated them. I went on some weekend club rides to push my limits and get stronger which it did but those longer rides of 30 miles were really hard the last 8 to 10 miles felt like torture, arms shoulders and butt OMG the butt. After a few months I realized I was no longer thinking about the seat because it was not bothering me anymore and the bars were much better but still uncomfortable. I decided to get a bar extender for a more upright position and boom I was a happy camper. The bike (Giant Escape City) proved not to be a comfortable ride for more than 15 to 18 miles but for my 6 mile one way commute it was great.
I just bought a Kona Sutra SE and after getting used to the flat bars I am having to get used to drop bars, very different ride, so again I shall resist the temptation to start changing parts on a well thought out bike until I have ridden it long enough to know for sure what works for me and what does not.
Being older I assume it will take me longer to acclimate to new things physically so I try not to rush it.
My first 25 mile ride on the Kona illustrated to me why I do not want to get rid of the drop bars, going down a hill on a rough unmaintained asphalt road was terrifyingly a terrific, the prospect of hitting something unexpected at speed was high but the bike wanted to go fast and I did not want to slow down so I went to the drops and my confidence and control was much better, hen you are hauling ass the drops are the ticket and who does not want to take advantage of a hill on smooth tarmac, really I'm not that old yet.
Still not sure if I may want to raise the bars a little they are already almost even with the seat so I shall wait and see.
I made a 50 mile ride longest ever for me and honestly towards the end of the ride I was feeling pretty good I could have gone for more but I was running out of daylight.
So for me, I say the bike matters a lot, I would never do a 50 mile ride on the Giant.
I also think that many new riders start looking to change things prematurely, the riders lack of conditioning my be the major culprit at first and until you address that issue you really should not start tweaking things, changing a seat before you break in your butt is not going to solve that issue. If you read to much you are going to be convinced you need a new something or other.
I am not at all sure I need to raise my bar but I have decided that I need to get just a little closer to the hoods so I am fixing to move my seat a little forward go fo a ride and test that theory. I don't thing it is possible to ride for hours and hours and not experience some discomfort, even in my car on a long day trip sometimes I just want to get out walk around and stretch a bit.