Some questions about running

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This has probably been talked about way too many times, but here I go and question again. Actually a few questions.

After almost 2 years no running thanks to ACL issues. Now 2 surguries later, I was given the go ahead to jump back on the treadmill. Before my operation 2 years ago, I finally got to be able to run 40-50 minutes at 9km per hour. Not much, but I was proud. Back then I never had issues with my hips, but since I started running a month or 2 ago, I have had left hip issues.

So my first question is this, when people say go slow when you run to begin with, is it only to do with getting your fitness up? Or is it so that 'running' muscles get used to running? Or is it both? My sister says that my hip issues sounds like a strain from too much too quickly, (shes not a doctor, but a PT). To be honest I did jump straight on to doing 15 mins at 8km, then 4 weeks later I was up to 8.4 km per hour, is that too much too quickly? BTW my knee has no problems now!! Its stronger than ever...as long as I wear my brace lol.

Second question is, whats your advice about speed versus distance? Should I care about one more than the other? I was told that its better to run further than to get faster if you want to burn calories and fat.

I like to interval train, I kinda get bored running the same speed after 10 minutes. ATM I am running 8.4 km per hour, then I go up to 9 km per hour then for a minute, then 2 minutes I go up to 10km per hour. I can do 3 km in 20 minutes, I could probably running to 30 mins if I wanted too. When I am done with this run (15-20 min) I am out of breath and my heart is pounding.