I am hoping to cycle Lands End to John O'Groats next year, or the year after health depending.
I have a few long term health issues, mainly respiratory, that have been compounded with a recent bout of pneumonia, and what the doctors believe is long covid. As such my cycling speed is currently sat at just over 6.2miles per hour (which is up from last month's 4.5miles per hour

I want to try and do the trek in 2 weeks so I can do it around work, but 2ith my current speed I'd be doing 17 hour days which just isn't feasible.
Has anyone trained up for this sort of challenge from such a slow start and could offer any advice?
I'm currently cycling every morning before work, between half an hour to just under 2 hours depending on how I'm feeling, alwirh a recent 3 hour cycle this Saturday just gone. I have a few long cycles planned in August that will be 5-6 hour days that I'm hoping I can work up to.
My speed is slowly improving as the weeks go, but I'm worried that I won't ever be able to get to what I'm told is a good average speed of 15miles an hour, it just seems so far out of my reach right now and I could do with ideas for how to keep progressing.