I did it last year for the first time. Thoroughly enjoyed it and was glad that I’d done a decent amount of training for it, which meant I could enjoy the ride. I really wouldn’t want to do it on minimal training.
As said above you need to include a lot of hills in your training. Most of my rides leading up to the Fred had around 1000ft of ascent per 10 miles and I was getting up to 80 and 90 milers in the preceeding month or two.
Hardknott after 0 miles would be hard enough but after 100 is frankly ridiculous. A great feeling when you make it up without stopping though. I’d say about 80% of people walk at least some of it. There are many climbs on the route though – I remember Honister being particularly nasty!