England missed out on the title on points difference to Ireland but won four games in a row after losing their opening match in France.
"I've always believed that (we can win the World Cup) and I've believed it from minute one," Lancaster told reporters on Sunday.
"It has been a steep curve and we have had to do it quickly," added Lancaster who took over as coach two years ago.
"We have been through our period of transition. I look at other international teams with a lot of over-30s playing for them and think 'they are going to have to go through a period of transition at some point'.
"We've been through that and have got a very young side. This team will be around well beyond the next World Cup and way through to 2019."
England lost 26-24 in France after conceding a late try but they beat Scotland 20-0, Ireland 13-10 and Wales 29-18 before finishing up with a crushing 52-11 victory in Italy.
They ran in seven tries against the Italians after scoring seven in the previous four games and Lancaster is under no illusions about where England need to improve most.
"Attack," he said. "There is still room for a 10 percent improvement in every area. There is a long way to go but this Six Nations has been a good step forward for us.
"I'm never satisfied and always want to keep improving."
England play three tests in New Zealand in June before the November series of internationals against the All Blacks, South Africa, Samoa and Australia.