I'm in Shimagnolo's camp - I doubt very much that bonk is an issue for what is still a relatively short ride. Even if you're on a low-carb diet, you should have had enough energy stores to do the ride. That HR number is extremely low considering you were active at the time. An easy ride for me has me running at 115-130 bpm and although it may drop soon after getting off the bike, it's at 80 for a long time afterward. At 49, you were asphyxiating!