The three 12-hour riding shifts in as many days have now raised over £340,000 for the NHS.
The 2018 Tour de France winner was riding on virtual riding platform Zwift and streamed live on Thomas' and Team INEOS' Facebook pages.
'I just didn't have it' - GB's Thomas reflects on disappointing Olympics
"No exaggeration, the last two hours were the hardest I have ever had on the bike - I could barely sit down," said Thomas.
"I am going to have a week off to recover from this. It was 36 hours in the saddle over three days which would normally be a big training week for me.
"The longest ride I have done on the road is eight hours and 20 minutes in a group and that was with a coffee stop.
"On a turbo the longest was three hours and there is a lot of difference from that and riding outside where you have the scenery and downhill."
Thomas excited that Tour de France is going ahead this year
‘What have I done in a previous life?’ - Thomas after another crash at Olympics
'My head's all over the place' - Thomas rues crash in Olympic road race