This may be old hat to most, but I've recently helped someone with this problem, so it's worth going over it again for anyone who just doesn't know. It's a craft point, and it's really simple.
Viewpoint, or being in the character's head, gives a story immediacy. Even though you are writing in the past tense, it feels to the reader as if everything is happening now. Your objective is to pull the reader in so that they are seeing, feeling and thinking along with the character. Then you can inject "information" about actions and it doesn't interrupt the flow or the viewpoint.
Reduced to simplicity, you write just as if it was "I" but substitute "he" or "she" and add the character's name every so often for clarity. Thus:
First person: "I raced hell for leather down the alleyway, shoving aside without apology a kid wearing a hoodie. I could just see the black-coated figure ahead of me. If I could only get a spurt on, I'd overtake him in seconds."
Third person: "He raced hell for leather down the alleyway, shoving aside without apology a kid wearing a hoodie. He could just see the black-coated figure ahead of him. If he could only get a spurt on, he'd overtake the guy in seconds."
Almost exactly the same, except for identifying "the guy" instead of "him" to save confusion because we already have several "he's" along the way.
Frankly, that's really all there is to maintaining viewpoint inside the character's head. Too much is often made of viewpoint, but if you stick to this really simple formula, it's easy. You've got action, thought,and characterisation in three sentences. When things calmed down a bit, you could add feelings too, along with using the other senses.
Writing doesn't need to be complex, honestly. Stick to simplicity and you can't go wrong.