There are some injuries that the running community seem more prone to than any others and one of these is achilles tendinitis. The achilles is the large tendon that runs down the back of your lower leg and is the largest tendon in the body. It connects your calf muscles to your heel bone and is used when you walk, run, climb stairs, jump, and stand on your tip toes. Although the Achilles tendon can withstand great stresses from running and jumping, it is also prone to overuse and degeneration sometimes resulting in tendinitis. Tendinitis can occur at anytime, even in people who aren't regularly active but more commonly results after years of overuse (e.g. long distance runners and sprinters). It's more likely to occur with a sudden increase in the amount or intensity of exercise activity or from having tight calf muscles. If you do develop tendinitis, rest and ice along with anti-inflammatory medication (always consult your doctor before use) are the first steps in recovery. But there are some exercises you can do to help.
Calf stretches : Lean forward against a wall with one knee straight and the heel on the ground. Place the other leg in front, with the knee bent. To stretch the calf muscles and the heel cord, push your hips toward the wall in a controlled fashion. Hold the position for 10 seconds and relax. Repeat this exercise 20 times for each foot. You should feel a strong pull in the calf during the stretch.
Heel drops : Stand at the edge of a stair, or a raised platform that is stable, with just the front half of your foot on the stair. Lift your heels then slowly lower your heels to the lowest point possible. Repeat this 20 times with both heels together and also with each heel individually. This exercise should be done in a slow, controlled fashion and can be made harder with the addition of a weight in each hand.
But of course, prevention is better than cure so try to avoid sudden increases in your running distances, particularly if you're moving from road to trail running, and always do your stretches after running!!
In addition you could try three sets of 10 repetitions of these exercises for strengthening your achilles tendons.
Heel raises : similar to the heel drops above but done on solid ground. Raise yourself up and down on your toes, first on both feet and then on one foot at a time
Towel stretch : sit on the floor, legs extended with a towel draped around the soles of your feet and push your toes against the towel
Leg press : using the leg press machine at your gym or a strong resistance band, using both legs together and each leg separately