i started on the treadmills a few years ago but got bored after i went outside a few times. as previous advice take it careful. Are you looking to go road running or into the sticks and trial run. i went into trial running and found it kept everything interesting and really had my focus on the surroundings.
its not an issue to mix both but you may find that you will get bored of the treadmill the more you take to going outside.
I did this run last year and loved the atmosphere but the hill stung me near the end and had to walk for abit and my calves felt like someone had poured concrete inside them. but im really looking forward to it now even more so after having the e mail to say my number been dispatched.
good weekend? what stretches have you tried for you calves ? have you tried just like placing your legs has if you walking so one in front of the other and leaning forward bending the front leg and keeping the back foot firmly on the ground you should feel the calve pulling abit hold for around 6 seconds, it may help
Victoria as MM and Sinbad said just break into it slowly by giving yourself little targets even if its run to one lampost then walk to the next etc etc build yourself up slowly and then running 2 lamposts and walking one, you may find all this will ease up on your coughing fits as your body adjusts to this kind of exercise.