Last week I started running. I've been cycling for 3 months now so I'm in the best aerobic shape of my life. But I've read that aerobic fitness doesn't necessarily carry over from one sport to another. I expected it to take me a while to get comfortable running.
My plan was to walk/run for 20 minutes, 3 times per week. To start I decided I'd warm up by walking for a couple of minutes. Then I planned to run for 30 seconds and walk for 60 seconds. I decided that if I needed to walk for more 60 seconds then I'd do that.
I got my heart rate monitor on Friday, March 24. Sunday, March 26 was the day of my first run. I stayed out for 20 minutes. I had to extend a couple of my walk intervals to 2 minutes, but for the most part I was able to stick to the plan.
I felt pretty good after my first run. My hips and knees were a little sore for the next couple of days. But I kept icing them and they improved fast.
On Wednesday March 29 I did my second run. It was very cold that morning, but I had my coat on, my hood up, and my gloves on, so I stayed fairly warm. The only real problem was the wind was blowing into my face on the second half of my run. My face got pretty cold and my nose ran, but other than that it was a great run. Later that day my hips and knees started aching again. This time my left Achilles tendon got a little tender too.
A few years ago I had a minor bout with tendonitis in my left Achilles tendon. I had started walking. At the time I was 300+ pounds and I would get to the point where I could walk faster and then I'd over do it. At first I thought it was just a muscle ache. I kept walking every day thinking the pain would go away once I got used to the exercise. But after about a week the pain kept getting worse. It got to the point where I was noticeably limping. That's when I decided to do some research.
I self-diagnosed it as tendonitis. From what I could find that was the least of the problems I could have. The recommended treatment was to ice it and elevate it regularly. Most importantly, I needed to rest my ankle. I didn't worry about my diagnosis being wrong because the treatment for tendonitis wouldn't worsen the problem if it turned out to be something more serious. I decided if I didn't notice an improvement I'd go see a doctor.
Icing, elevating, and resting helped. Within a week my limp was gone. Within a couple of weeks most of the soreness was gone.
Anyway, I'm very aware of my Achilles tendon now. I also pay better attention to other aches and pains too. I now know the difference between muscle aches and sore tendons 
On Thursday, March 30, I got the speed and cadence sensors for my bike. I originally planned to run again on Friday. On Friday morning I felt like I was physically up for a run, but I really wanted to ride my bike to test out my new gadgets. And I figured the extra break from running would give my body a few extra days to recover from my first 2 runs.
I didn't run again until this Monday. I hadn't planned on increasing the length of my runs or the amount of time I spent running. But when I got out there I felt good enough to run for 45 seconds and walk for 60 seconds. I also decided to stay out for 30 minutes. The extra 10 minutes and the increase in my running time meant that I basically doubled my distance. The first week I covered just over a mile in 20 minutes. On Monday I went about 2 miles in 30 minutes.
I felt very good during my run. But later that day my left leg was pretty sore. The tendons in my hip and above my knee were sore and my left Achilles tendon was also tender. My left foot was also sore. By Monday night I decided that I'd probably need to skip the run I planned to do this morning. But by last night I was feeling pretty good. In fact my daughters and I ended up going out to play a little tennis. The weather was beautiful and the sun now stays up until almost 8pm.
I felt good after tennis but I wasn't sure how I'd feel this morning. My legs were a little tender. I iced them before bed but I thought I'd probably end up skipping my run this morning.
But when I got up this morning I felt great. My legs weren't sore at all. I threw on my running gear and hit the road. I felt great until I was almost home. Then my Achilles tendon started to feel a little tender. I decided to walk the last 3 minutes home. After I got ready for work I iced my ankle for a while. I've felt pretty good throughout the day. My left hip is sore, but not as bad as it has been. My Achilles tendon is almost completely normal. There are a couple of positions where I can feel just a tiny bit of swelling in it but that is only because I'm paying very close attention to it. I iced my hip and ankle while I was at home for lunch. I'll ice it again tonight.
I plan to run again on Friday morning. At this point it seems like I shouldn't have any trouble doing that.
I did notice that my aerobic fitness didn't translate to running. On my first two runs my heart rate was very high considering my pace wasn't much faster than a brisk walk. But this week I noticed that my body is adapting much faster to this new stress than it did when I started cycling. I expect I'll see the same thing when I start swimming too. But I hope my body adapts quickly to swimming too.