A rule of thumb I learned from runners, that's always worked for me, is to train to 30% of the hardest pace you want to do. So when I had a goal of riding 600 miles/week on a cross-US tour, I trained the winter before to 200 miles/week, and that worked beautifully. Similarly, on some long distance treks (AT, PCT, CDT), I wanted to walk 20 mile days consistently, I just trained to about 7 miles per day. It still took time and commitment, but it worked great, no training injuries.