[header] How to build up to long runs [title] Do one long run a week. [step] Take good note of that-don't make all of your runs longer. If you've been running 3 miles (4.8 km) a day (for example) 3-4 times a week, don't suddenly make all your runs 4 miles (6.4 km). 
Just pick one day a week to go longer. [title] Gradually increase that long run by half a mile to a mile each week.