Pietersen missed the last three Ashes Tests
Kevin Pietersen said he was feeling physically and mentally refreshed after joining up with the rest of the England squad in South Africa.
Pietersen has spent the last three months recovering from surgery to an Achilles tendon, an injury that forced him to miss the last three Ashes Tests.
"I'm feeling good and I'm feeling fresh mentally," said England's star batsman.
"I'm certainly not going to rush things, I tried that a while back and I got an infection in the wound."
On Wednesday, the 29-year-old faced several net bowlers as well as spin coach Mushtaq Ahmed, and also ran between the wickets.
It was his first outdoor net session since July, and after arriving for the two-and-a-half-month tour to the country of his birth on Tuesday, he said he was raring to go.
"The fitness is not too bad," he said. "I've done some rigorous training over the last six weeks, I've done a lot of work at Lord's in the last week.
"I had a bad day on my Achilles on Tuesday, but I think that was just purely down to the flight. I've woken up feeling really good.
"I enjoyed the break to refresh my batteries, to get myself right and ready to know that I can compete again and want to compete again.
"But I hated getting injured because I wanted to continue playing, to be a part of what was a successful summer. But now to be back in an England tracksuit is great."
Pietersen has endured a difficult year, which began with him as England captain, a position he subsequently relinquished following a very public row with former coach Peter Moores.
His Achilles problem meant he missed out on England regaining the Ashes before an infection caused more complications that caused him to miss the Champions Trophy.
"I think it's common knowledge what got on top of me during the first half of the year, things that happened back in the UK and the pressure was just mounting up," he said.
"Anything I seemed to do seemed to be a mistake and it was a great test of character.
"I've just treated this break as something to get my head right, to get myself back in love with the game.
"I hated missing fixtures for England, I really wanted to be part of a successful Ashes campaign.
"I missed out on a lot of cricket that I would have been playing. But I used the break to try and get my head right and to get myself ready, fit and raring to go again and I feel good now."
England completed their warm-up programme with a disappointing four-wicket loss to South Africa A on Tuesday.
They play the first of two Twenty20 internationals on Friday, followed by a five-match one-day series and four Tests.
I just don't want us to make the same mistake we made in 2005 where we sort of went on a downhill slope after what was a fantastic summer
Fit-again Kevin Pietersen
And Pietersen is hoping they can continue the momentum of the Ashes, unlike their post-2005 performance, which saw them lose in Pakistan.
"I think the team looks like they're really on fire to do some good things out here in South Africa and to be really competitive," he said.
"I'm very encouraged to hear all the positive things that have been said to me by the players over what's happened in the past couple of weeks.
"I just don't want us to make the same mistake we made in 2005 where we sort of went on a downhill slope after what was a fantastic summer.
"I think we're all encouraged and all really keen to make sure that this is a really successful campaign post such a successful Ashes as well."