Wales' record try scorer has made a full recovery from the knee injury which cut his Six Nations tournament short earlier this year.
That injury meant the former IRB Player of the Year missed Wales' clash with the Barbarians last month.
Williams, like the rest of his international colleagues, is half-way through the first of two RWC skills and fitness training camps in Poland, under the guidance of Head Coach Warren Gatland, backs coach Rob Howley, forwards specialist Robin Mcbryde, skills coach Neil Jenkins and defence expert Shaun Edwards.
"This is the first proper running I've done since the injury in March," said the Ospreys wing, who, like the rest of his colleagues, is packing at least two normal day's training into one 14 hour period each day on the camp, with the aid of Cryotherapy chambers.
"But I'm back doing everything the rest of the squad are doing, working hard on both skills and the conditioning side of things here in Poland, and I'm feeling good. All I need to do next is catch-up with them because the boys have been training really hard.
"The cryotherapy will kick-start that process though and, contrary to some of the other boys, I actually don't mind it.
"We are down to -140C, which is severe cold, that's low enough to have actually given some of the boys cold burns on their skin.
"But it is mentally tough as much as anything else, you have to tell yourself it's doing you good.
"It's colder than I've ever been before, but for me I'd rather be cold than be cold and wet as well in the ice baths."
At 34, Williams claims he is still learning new tricks and refuses to take selection for his third World Cup for granted.
"I'm always looking over my shoulder, we have some good youngsters coming through, but I'm not ready to let go yet," continued Williams.
"I'm playing catch up at little bit at the moment because I obviously haven't been able to run since March, but I know I can't relax for a second.
"I've got it all to prove again now and I hope to be able to play in each of those summer internationals (England home and away and Argentina home) and be able to stake my case.
"No one goes to a World Cup on reputation and I wouldn't want to be there myself if that was the case.
"We are looking to peak as a squad towards the end of August, physically and also as a team, and I want to make sure I do that personally as well."
Williams has been working on a one to one basis in Spala with athletics expert Frans Bosch (the biomechanics professor currently seconded to the squad) as well as taking a full part in the fitness regime designed by WRU head of physical performance Adam Beard and daily skills sessions lead by coaches Gatland, Howley, Edwards, Mcbryde and Jenkins.
"I'm back and fit again but I want to be quicker than I've ever been and we think that is possible," added Williams.
"It's strange but, at 34 years old with 79 caps, I'm realising that I haven't been running properly and that if I can pick up a few different things I can be quicker.
"That's brilliant news for a rugby player, everyone wants to be quicker, from the forwards through to the backs, but if I can achieve that at this stage in my career then I'll be really happy.
"But I've got plenty of competition with the likes of George North doing the same training and apparently testing pretty highly.
"We haven't done the sprint test yet and I don't think I'm quite up to top speed yet myself after the injury, but it will be interesting to see who is the fastest.
"I'm not going to make any claims though, I'll do my talking on the track when the time comes."
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