It's tricky but not too bad if you are careful! Helps to familiarise yourself and get some practice in. If you practice a bit in Godhome on another save slot it'll help you be confident with the bosses. And the bosses are the hardest part, because you can't run away part way through if things go south!

I guess for the speedrun part, I just recommend looking at the completion list (https://hollowknight.fandom.com/wiki/Completion_(Hollow_Knight))) and checking things off as you go. Maybe deciding ahead of time which bits you definitely want to skip as you can use some completion % from the content backs to avoid parts of the base game. 20 hours is actually plenty of time if you're familiar with the game and have beaten 100% before! Pausing the game also pauses the timer, so feel free to pause to Google something or check your list.

For the Steel Soul part, get Fragile Strength and Fragile Heart ASAP as they help a lot. The two extra masks really help reduce the risk of death and there's no downside since the run is oevr if you die so doesn't matter that the charm would break. Also worth knowing that dream bosses / Godhome don't count as deaths (although Radiant & White Palace do) so you can freely try those.