Slowing down may be certainly related to the new joints and extra weight. Are you just slower or are you getting more winded over the same rides you did before? If the later is true I would see your physician and make sure it is not cardiac related.

I ride with a guy who is about 6yr younger than you. 2 weeks ago he was complaining of some indigestion from a bad provolone sandwich. We cut our ride short that day. Over the next 3 days he rode about 70miles. When he was still complaining of indigestion. I recommended he see his physician which he did the next day and discovered he had a heart attack 4 days prior.

When he looked back over the preceding year he was getting slower and more winded and had to stop more often during the ride. He had attributed to increasing age.

He had a cardiac cath and 2 stents to open the narrowed vessels. We went riding last week (post stent). He did not get winded and we didn't have to stop at the normal places and says he felt better than he has over the past year.