I'm 61 and live in a mountainous area. Three times a week I ride about 40 km and once a week at least 100 km, sometimes much more as on a 400 km brevet.

Normally, during the summer months on a 40 km ride I can average at least 15 mph sometimes over 16 mph. In the winter my averages slow down, since I am riding part of the way in darkness turning a tire-driven bottle type generator.

I agree with Koffee that you've probably "overtrained." Last year, after riding two double centuries and three 400 km brevets along with my other riding in a five-month period, my average speed dropped.

I took a rest, not completely off the bike, but cutting intensity and the longer rides during the worst part of winter. My average speed started rebounding almost immediately. Even riding part of the morning in the dark, I still average over 14 mph and most often over 15 mph. I should be back to over 16 mph when the spring finally arrives here.

Go easy for a few weeks, then start back on building your intensity, though I do wonder about your not being able to complete a 1-minute interval. Rowan may have a point about your checking with a doctor. Since you've been cycling for a long time, a 1-minute interval should present no problem.