Now 60, he tells PEOPLE that "this one was injury-prone" – but don't count him out yet

Against all odds, Sylvester Stallone is back in the ring at age 60 – and, perhaps even more astounding, Rocky Balboa, the sixth installment in his Rocky series, has gotten warm reviews.

But much has changed for both Stallone and his alter ego in the 30 years since the original Rocky earned a Best Picture Oscar. Here, the writer, director and reluctant action hero talks to PEOPLE about his aching bones, keeping up with Arnold and why getting older hurts in more ways than one.

Was it hard getting in shape for this one? Were those bones creaking a little?
Without a doubt. This one was injury-prone. The joints wear down and you get inflammations of things that you didn’t even know existed. It became about living with conditions because nothing was going away. Every day was an Advil day. In all five Rocky (movies) put together I didn’t have this many injuries.

So show us your scars, Balboa. What did you hurt?
I broke my foot. I had a bulging disk in my neck. I shredded a calf muscle. It was so bad it had to be put in a cast. There (was) a lot of contact because I wanted it to be very realistic. We didn’t choreograph (the fight scene), so we ended up having the ragged edges, those awkward moments that make it real. In the other Rocky (movies) it was all kaboom-kapow-kaboom.

Did you have any idea when you became an actor that you would be training like an Olympic athlete at a time when most people would be thinking about retiring?
No. In the original Rocky, I didn’t have that spectacular a body. I was just this average-looking guy. But it was the beginning of the action era, and slowly but surely it developed. Then Arnold (Schwarzenegger) came along and really set the bar and I suddenly had to keep up with this guy.

Do you enjoy working out or is it hell for you?
I like it now, because it’s become a hobby as opposed to a chore. I feel bad when I don’t do it. I get these pangs of guilt. I’ve been doing it for 45 years, really.

Forty-five years working out and you wrote a Best Picture Oscar winner. You’re hardly a typical meathead.
I work on that too. I spend a couple of hours every morning doing head exercises – reading, getting on the computer. I have this thing I do called brain games. I try to remember numbers backwards, that sort of thing. I’m not too good at it, but I try.

But aren’t you the same guy who famously wrote the first Rocky in three days?
Three and a half – but I keep reminding people that it was not a shooting script. Only about 10 percent of it was salvageable. But I am a rewriter. I enjoy the process of writing. I can never understand those people who spend two years trying to get the perfect script one time out. That’s not writing, that’s waiting.

How long did this one take to write?
This one was complicated. It wasn’t working for the first four or five drafts that I had. Adrian was still alive and it was about saving the gym so children could work out there. It was more plot-oriented and didn’t deal with emotional turmoil the way the first Rocky did. So I thought I would plunge Rocky to his lower depths, and that’s how I devised the death of Adrian. That was the breakthrough that made the movie happen. I mean, a guy like Rocky would be at wit’s end if she died. She was everything to him. They had spent 30 years at each other’s side. It was the basic idea that life wasn’t supposed to be so hard. You make a lot of money, you get to the top and you say “Why aren’t I any happier then I was when I was broke?”

This sounds like your story, not just Rocky’s.
I think so. The character, though I try to distance myself from him at times, has become very autobiographical. But his way is a lot more street poetry then mine is.

As a rich guy living in Beverly Hills, what do you know about the poetry of the streets anymore?
There is no question that the rarified air can get to you. You get this strange entitlement, you lose touch. But things really started to slow down for me about 10 years ago and now I have a lot of time for introspection. If I had been cranking out films, very successful ones, I wouldn’t have done this. But I thought, this really translates to a very common dilemma for anyone who reaches the age of, say, 55. You reach that crossroads and society basically has deemed you passé. You’ve had your chance; now they expect you to just move aside for the next generation.

Which begs the question: Can you swear on a stack of Bibles that this really is the end for Rocky Balboa?
Sure. I couldn’t top this. I would have to wait another 10 years to build up a head of steam, and by that point, come on.