Definitely doable with training. I did HT550 a couple of times and just trained well and ate loads. First time (2014) I was slow at 7 days but dropped over a day off in 2017. I wasn’t that much quicker pedalling but was much more efficient overall.
Don’t bother with campsites. Any time stopped is time not moving. A campsite is a diversion and has too many luxuries. Minimise your stops, and maximise the value of them. Stopping to change a layer, eat, refill water bottles and pee can be done in one go.
Go light and be smart with your kit. I take an inflatable pillow because it means I don’t need to stuff spare clothes in a drybag. That means I wear my down jacket as part of my sleep system, so my sleeping bag can be lighter.
Make hay. If the going is good, keep going. A mile today is one you don’t have to do tomorrow. Don’t bother looking for a camping spot until it’s getting dark, otherwise you’ll be waiting to fall asleep.