When you are old and squishy, do NOT do this

About a month ago I decided it was time to get back in shape. And apparantly I thought you could do this whole getting back in shape thing in a few days. So I started exercising, a lot!
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I went bike riding, jogging, and to cap it all off I played in Zach's tennis class. That last night my ankle felt a little odd. Hmm....it has happened before, just walk it out.
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A few days later my lower back started hurting to the point where it woke me up and I was unable to move without extreme pain. Weird, but it is only when I lay flat on my back.
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Still in denial I am sure it will go away in time. But what IS that odd feeling in the back of my left knee? Like two lines running down the back of it and they feel so tight! What is that odd feeling?
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So for nearly a month I lived with a stiff ankle, back pain and a funky knee. Finally I went to the doctor who informed me, now get this, that you CAN'T get in shape in one week. He also told me, and this is really strange, that you are supposed to gradually work yourself back into exercising.
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I guess if you don't do it gradually you risk injuring a big muscle in your lower back, straining something in your knee, and hurting your Achilles tendon. Seems I did all three at once, all on the left side of my body.
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So now I won myself a 3 week, 3 times per week pass to the physical therapist.