He is a man much in demand - on the day that Horticulture Week interviewed him he had been asked to prepare a talk for the Government at short notice - but he appears to be tireless, talking passionately about how food security is needed by people across the world and how it might be provided.
"The world is entering an incredibly difficult time - with issues of climate change, water, soil, land pressures and biodiversity, just to name five," he explains. "The new world of the 21st century will not be one in the hypermarket model of cornucopia for the British consumer."
He paints a stark picture of the problems with the global food system: "There is currently massive overconsumption by rich Westerners, underconsumption in great swathes of the developing world and malconsumption everywhere." He adds: "In a world that will have to feed nine billion people in 2050, let alone 6.7 billion now, we're not going to be able to eat as the British currently do."
His greatest ire is directed at meat and dairy producers because of the inefficiency of producing masses of grain as fodder for animals: "We need more horticultural production and less dairy and meat production, and what meat and dairy we do produce must go back onto the hills."
Despite being an adviser to the Government, he is not afraid to speak frankly: "Government policy is still hopeless. It's fixated on being fed by other people. It's hopeless to be locked into the old empire, free-trade model, importing from outside the UK."
He has been saying for a long time now that one of the answers lies in local food production; in fact he was the first to coin the phrase "food miles".
While he spends a great proportion of his time advising the Government and the World Health Organisation, among others, in his role of president of Garden Organic he aims to make the wider global issues relevant to people involved in the horticulture industry and to the individual gardener.
"One of the reasons why I took the role on is because I want to help Garden Organic make better connections between the skills of nine million gardeners and food production. What needs to happen now is massive purchasing of allotment grounds."
Lang sees "a new era of urban and peri-urban agriculture", where food is produced in or near the cities in which it is consumed.
Earlier this year he took part in the Growing Food for London conference, where he and London Parks & Green Spaces Forum director Tony Leach asked parks managers to turn a corner of their parks into community garden areas where fruit and vegetables could be produced.
He acknowledges it would be difficult for the amount of land dedicated to food production to be increased in London because of building density, but he cites cities in developing countries that could teach town planners in the UK a thing or two. "We could learn from Kathmandu, where 40 per cent of food is grown in or near the city."
He has advice for landscape architects too: "They need to plant masses of fruit trees. The data on public health in Britain is incontrovertible. We need to eat masses more fruit and veg and only five per cent of the fruit we consume comes from Britain.
"The number-one thing to do now is to consistently plant fruit trees for the next 20 years and show people how to care for them. But they need to be replanted in a new biodiverse way, not just putting in five zillion acres of Discovery (the apple tree cultivar)."
He believes the national fruit collection at Brogdale could be utilised to achieve this. Brogdale is an example of the precarious existence of research and development centres in the current climate. Now securely managed by the University of Reading, last winter one of the options the collection was facing was being dug up and moved after Defra put it out to tender.
Lack of funding for R&D is one of the main areas Lang believes the Government should address: "The Government needs to rebuild horticultural research and development after making the decision to close much of it down. It has also got to take a lead on injecting sustainable criteria into horticulture.
"Thirdly", he continues, "it needs to address the issue of seasonality and carbon. We do not want year-round tomatoes if that means tomatoes are being produced in greenhouses during the winter, emitting large amounts of carbon.
"Fourthly the Government has got to give urgent attention to soil and water issues. What most of us working in food policy think is that we are coming to a crunch point.
"Climate change volatilities are now probably inevitable but controlling water and soil is going to be a critical issue."
And what should the individual be doing? "Be prepared for change. We're all going to have to eat differently. Less meat and dairy, more fruit and veg, using low levels of carbon and water. The big changes will occur, whether we'd like them to or not."
1970s: Farmer in Lancashire
1980s: Lecturer, Preston and Manchester polytechnics
1984-1990: Director, London Food Commission
1990-1994: Director, Parents for Safe Food
1994-2002: Director of the Centre for Food Policy, Thames Valley
2002: to date Professor of Food Policy, City University, London
2006: National Resources and Land Use Commissioner on the
Government's Sustainable Development Commission
2008: The Atlas of Food, second edition, with Erik Millstone