Trying to get back into speed training

I've been doing what we call "base training" for 4 months now. Just logging lots (for me) of mileage at medium effort (around 7-8 minute pace). I decided I'd better start introducing some speed training back into my schedule since I'll probably be doing some 5-8K races soon. Most of the running books I've read indicate that it's good to have months of pure base training, and then move into a period where speed workouts are introduced. So, on Monday, I was going to do my first short tempo run in a long time. I was going to try to run 3 miles while staying around 6-6:10 minute miles.

I started off fine, and felt good. All my base training had really helped, and it felt like I could keep going for quite a long time. It's just that, once I got to a certain speed, it felt like I couldn't go any faster. This is where speed work becomes necessary because I need to teach my body to run faster.

As I neared the end of my second mile, I felt a distinct twitch in my left calf muscle. Suddenly, I had significant pain on the outside of that muscle. It caught me off guard, and I stopped to avoid doing anymore damage. Perhaps I need to ease into such fast running more gradually.

The next day, I still ran, but had to take it easy due to soreness. I took the next day off, partly because of that, and partly because I had a sore throat. I'm feeling better today on both accounts, so I'll see how well I can train today.