Oh, you were serious about endurance advice........ sorry.... i joke.....
Increase slowly - yes. You don't go out and ride 100 miles in one shot when the longest ride you've done is 30.... that'll make you hate it. 10% seems fair enough - time or distance will work. On the MTB distance is affected by terrain so time is likely a better way.
However, you will reach a point that provides little further benefit. 50 miles on weekends on the MTB is sufficient. Throw in an occasional 75 miler. Once you can ride, say, the entire Grueller route at a 12-13 mph average and feel good after, you're pretty damn fit. Matt Lee isn't going out and riding 100+ days for training for days on end.... it will tear you down.
Which leads to rest. Google "Sweet Spot Training". You can also search the www.windermereroadies.com/
web forum and find a lot of comments there. it will outline how to get a HUGE bang for your training buck. SST can let you ride 5, 6, or 7 days a week and if combined with good food and good sleep, will make you strong like bull! Realize, we are average, we have a job, we have limited $$ for coaching - work with what is available.
food is personal. some like powders and gels and others like solid food.... up to you (or better yet, up to your stomach). Ultra distance will almost dictate no powder/gel and demand junk food. Sucks for sure. but when you are surviving off convenience stores, fresh fruit and veggies are rare. You can't carry enough food with you for a multiday ride to be "good" food - ain't gonna happen. You will need 8,000 - 10,000 calories a day for a fast hard attempt at, say, a TNGA. You can't carry lean chicken, fish, potatoes, rice, fruit and veggies ..... and sometimes you won't have a restaurant option. Learn what gas-station food works well BUT DON'T MAKE THAT YOUR TRAINING FOOD. Bread of any sort (sandwich, pancakes, cupcake, etc) is fast to pick up, quick to consume and turns to sugar in the blood very fast. High fat levels in food are also great endurance energy sources. Milk, bacon, olive oil (for real), fatty nuts, fried chicken finger.... very dense in calories. But only consume that crappy stuff while racing or multiday bikepacking when needed.
and back to my original statement.... "lack of sense to stop". I was somewhat joking, but not really. To race TNGA, RAAM, TD - you can't quite when you get bummed out. you must be resolute in your conviction and be able to manage the inner voice yelling to 'quit and go home to your nice warm bed.' You must not turn "OFF". Unless continuing is going to physically hurt you - injury or illness - you just always move forward.
My worst enemies this june = My mind and injury.