Yeah, 'Ultramarathon Man' is a great choice. I've read it myself and it's definitely inspiring. If you're looking for something a bit more focused on the mental side, you might want to check out 'The Champion's Mind' by Jim Afremow. It's got some great strategies for building mental toughness and overcoming obstacles.
Ugh, achilles tendinitis is the worst. I had it last year and it took me 6 weeks to fully recover. In addition to rest, ice, and stretching, I started doing eccentric calf exercises and that seemed to help. Also, made sure to gradually increase my mileage and intensity when I started running again. Fingers crossed for you, mate.
Yeah, 2-3 20-milers is a good range. For me, it's about listening to my body. If I'm feeling strong, I might do an extra one. But if I'm feeling fatigued, I'll stick to two. It's all about finding that balance between pushing yourself and not overdoing it. And let's be real, there's no one-size-fits-all answer here. It's all about experimentation and finding what works for you.