Thanks chasm, Looigi, rf and all who responded. Very helpful and much appreciated.
Learned how to get more details from ride (love this forum), and found average HR below 125 first 2:15; 125-150 from 2:15 to 4:00 hours. Then came East two hard miles where HR went from 145 to 196 over about 10 minutes.
HR rose from 176-196 during last two minutes. Yes there were two brief spikes at end to 196 and again to 179. Not sure but about a minute in length as rose up, down and up again to 179.
Temp 95 during this time on level road. Cadence at about 90 during these 8 minutes and for entire ride.
So.....yes two seeming spikes but still high HR over about last two minutes. Otherwise HR around 170 which is likely my proximate max based on riding Miami hill--bridge to Key Biscayne.
Not sure how long a spike is and Garmin Connect doesn't allow exact times
AFAIK.
Thanks again to all. Feeling better already.