You asked for thoughts, here are a few:
At first glance, I thought you wanted to ride this weekend...put right by your next post about sometime in june - a much better option IMO!
if you are happy to do the the Petersfield to Shoreham leg, then this is very do-able.
You do not
need to go all the way down to Shoreham. You can drop down off the SDW into Steyning, stay overnight there - both B&B and hotel options, would recommend the former - and pick up the Downs Link from Steyning.
Folk who want the full 100 mile experience would opt for MTB. Certainly more suitable. However, gravel bike could easily cope. With one caveat - no rain in previous 7 days! I ride a small - very rough in places + steep gradients - section of SDW near me on an old BSO with 700 x 38 tyres, they are hardly MTB knobbly, probably over-inflated for the surface. All cope fine - apart from rider sometimes!
This is the whole point of the Downs!
If you can manage Day 1 reasonably, then Day 2 will be a breeze. Dull even. And boring after about 10 minutes. Or at least the section from Partridge Green to Southwater. (The section from Shoreham to Steyning is more interesting).
It is very flat. I recognize this might be appealing after Day 1's exertions!
---
Very true.
True, but much of it is wide where progress is not slowed hugely. Also used by equestrians. Livestock can also cause obstacles but they are generally benign and out of the way.
Refuelling may also be taken off the SDW but this can be time consuming. Planning required.
---
Indeed, but
not to be relied upon. Sometimes they are out of order. Re-fill bidons before you need to.
---
Am very familiar with Amberley to Steyning area in general and a little less a bit beyond in all 4 compass points. Any specific queries bike related or otherwise, ask away.
And once you have enjoyed your easy weekend, you can consider a proper challenge - the South Downs Double. See
here.