The Everton centre-back suffered a small hamstring tear shortly after the 1-0 Premier League defeat at Chelsea on February 22, having previously struggled with a similar problem at the turn of the year.
That injury caused Jagielka to pull out of the Three Lions squad for the 1-0 friendly win over Denmark at Wembley in early March.
The 31-year-old ended up spending over two months on the sidelines at Goodison Park, eventually returning in the 3-2 home defeat by Manchester City just over three weeks ago.
Jagielka played 66 minutes of that encounter and then the full 90 at Hull the following Sunday - his only competitive action in three months.
However, the defender feels last week's World Cup training camp was hugely beneficial in terms of his fitness and speaking at an FA media day on Tuesday, he believes he is on track to be ready for the Group D opener with Italy in Manaus on June 14.
"It was an important week last week," said Jagielka, "getting out to Portugal to get a few more sessions under my belt, and we had a really good session today.
"I do not need to be 100 per cent fit this week. It is more about building up and getting match practice before we go out to Miami, hopefully timing it perfectly for the opening group game."
Jagielka is the most experienced of the centre-backs in Roy Hodgson's squad and would be tipped by many to line up alongside Gary Cahill at the heart of the defence against the Azzurri, particularly as England have not lost a game when they have started together.
However, despite making seven appearances in qualifying, he faces stiff competition from Chris Smalling and Phil Jones for his place.
"I am not guaranteed to play but I am in a better position than I was two years ago," he added, referring back to Euro 2012 when he was an unused squad player.
"I have played a decent role in the qualifying campaign to get us to Brazil and it would be nice to get some game time and be part of a successful team."
England host Peru at Wembley on Friday before heading to Miami for friendlies against Ecuador and Honduras.
"It is all about momentum," said Jagielka. "If we can get some decent results in the friendlies we have got before the tournament, confidence will be high."
He added: "People may be playing down our chances but momentum is a fantastic thing in football. The lads are really positive."
Jagielka believes his game has come on a lot since 2012, partly due to the fact that he was named Everton skipper at the start of last season.
"Being made Everton captain this year forced that on me a little bit more," he added.
"I am quite loud and vocal on the pitch. It is important we have leaders in all different types of areas."