Hello Peliroja
The first thing I'd like to ask is: what do you consider to be a slow ride? Having experienced first-hand your whizzy cycling ability I reckon your version of slow is still faster than wot people on here are recommending
I'd echo much of what teethgrinder said...(and a lot of other good advice here too). If you've been really poorly it can take a looong time for your body to recover, especially if you're working long hours and beating yourself up for feeling a bit below par. If your body is telling your brain "please make her stop" but you're fighting it, it can make for a weary situation.
<long indulgent post alert>
When I moved from London to Devon I lost the plot completely for a bit, was poorly, depressed, got very unfit, lost my bike mojo and then made it even worse by forcing myself to get out and ride.
End result = hated my bikes and felt completely out of sorts with the world. The final straw was freaking out 20 miles into the middle of a 30 mile ride (about 9 months after being able to do Dun Run style distances
fairly easily) because I wasn't sure that I was going to be able to get home. My confidence was shot to smithereens.
I had a couple of weeks completely off the bike. Then I had blood tests and blood pressure tests which were all ok, so far so good. Then I forced myself to stop thinking about what I'd been able to achieve, and started going out for pub rides, cake rides, picking-up-the-paper rides and gradually started extending the distance - always on circular routes so I was confident that I could get home. If I felt like staying in bed, I stayed in bed.
I also limited myself to only using the middle ring for a while so I couldn't blast along and use up any energy too quickly.
The end result was that I slowly got back to form and the following year had my best year of cycling so far, ever.
</end of indulgent post>
So, if you think it will help, chat to the doc, slow down and try not to give yourself a hard time.