I found myself in similar situation not long ago.
Some friends on this forum helped with their experience which is accessible through the following link.
I am feeling quite good these days and the key has been:
Cutting down the morning sleep
Joining a gym, a pool, and a volleyball group
Committing myself to write certain number of hours on my new book that gives me a sense of purpose
Joining a few social groups and activities that help me feel less isolated
Joined a driving school and studying to get a driving licence for heavy vehicles
I shared how I felt with 4 friends (2 couples) and they invited me to spend one full day and night with them on alternate weeks so I enjoy enough human contact on a weekly basis.
Retired early and baffled!