I can so relate to the acid thing... From my (albeit brief and recent) experience, I've found that once you go over that 20 mile mark and begin to burn, it's for one of two reasons, and sometimes both. The first is 'pressure', where your soft bits are being squashed onto a saddle that does not suit you anatomically. Try different saddles, particularly those with either cut outs or gel inserts. I now have the latter, and love it. Make sure that when you ride, you are wearing bike shorts with a good chamois - riding without will bring you grief.
The second 'cause' is friction, which is all about your shorts. No matter how well fitting they are, if you are senstive in that area, you will rub yourself raw once you go over a certain distance. You need to lubricate the chamois. I know some women say 'ugh' to having a viscose product on their private parts, but honestly, I just don't notice it any more, and I've been doing plus 50m without any chafing since I started doing it. And don't go spending $$$ on expensive specialised butter and the like; go to the supermarket and buy tubes of pure lanolin for about $5. If it's safe for babies, it's safe for me. Just slather it over the chamois, and off you go! Just remember, when you get back from the ride, get those shorts off straight away, put you and them in the wash, and Bob's your uncle! Let us know how you go.
A
PS The added bonus of lanolin is it keeps the chamois super soft!