I try not to lecture people about running in sandals, but please start slow! (Individuals alway vary, you must listen to your body!) Here's what I recommend:
Week 1- See a doctor & get a physical.
Three 20 min walks in sandals (adjust sandals as necessary during & after). Occasional jog around the block to see how they feel and where they need adjustment.
Week 2- Three 15-20 min slow runs (you may get hot spots, and it will give your skin time to adjust w/o getting massive blisters).
-Soak any blisters in a bucket w/ equal parts water, yellow Listerine, white vinegar
-Rub on O'Keeffe's Foot Lotion before bed as you'll get very painful skin cracks as your feet adjust
Week 3- Add 10% each week. 20 mins becomes 22 mins.
Week 4- Continue this pattern. 22 mins becomes 24 mins.
Week 5- You can go to 5 days per week if training requires it. For general fitness 3 runs per week is totally fine. Follow the 10% rule until your reach 45 mins. Very few people need to go beyond 45 min runs.
Long Runs typically follow the 20% rule. A runner doing 40 miles per week should not exceed 8-10 miles for their weekly long run. As such, very few people end up running longer than 45 mins. Those that do have followed the 10% weekly distance rule and are fully acclimated.
It's usually a good idea to rotate w/ shoes until about 6 months has passed. It can take people 12 months to fully convert to sandals. It also reduces blisters. Put a dab of vaseline on the hot spot before you put on your socks and shoes.
People sometimes get injured because they get excited and they want to get fit too fast. The margin for injury is very small in sandals. However, they give much faster feedback than shoes. LISTEN TO THAT FEEDBACK!
I've been a cross country coach for 26 years. Almost all running related injuries can be treated w/ 6-8 weeks of rest followed by starting over w/ the above advice. I cringe every time I hear that a customer is 'getting ready for a big race'. Remember, it's for a lifestyle not a race!
I believe sandal / minimalist running is a lifestyle thing. It's a little bit like when people discover yoga or meditation. A little bit goes a long way until years has passed and you're fully adjusted. Please go slow!
"You are what you do every day."