Car parks were full of day-trippers sunning themselves in this unexpected break in the foul weather. Among them was a large gathering of bedraggled runners with red faces and legs caked in mud. They were orienteers who had come from all over Britain to take part in the November Classic, an annual event held here for more than 30 years.
There were scores of them, stretching their lycra-covered limbs and sucking at the air noisily. Most were accompanied by partners, children, dogs, and in-laws who produced gargantuan feasts of soup and sandwiches from car boots. Others wandered among the windswept stalls selling sweat headbands, replacement insoles and energy drinks.
Orienteering is now extremely popular both in the UK, with roughly 10,000 people taking part each year, and worldwide, with countries like China among the most recent to take it up. One runner at the Classic said men especially get an enormous ego boost from directing themselves through a challenging course and proving to their wives that they can navigate "in the wilderness".
"I suppose it stems back to when we were cavemen," he said. "Of course, we are so pleased with ourselves after a day orienteering that we drive home and end up missing the turning off the motorway."
As I changed into my wild weather gear, however, I was still not sure this was the best way to spend a Sunday afternoon. It was too reminiscent of school cross-country tournaments led by sadistic teachers. We pupils would brave sub-zero temperatures in our shorts while they screamed at us from inside fleece-lined overcoats, accusing us of being sissies.
Peter Robson, a member of the Southampton Orienteering Club who organise the event, was determined to convince me. We set off at a light trot on the Wayfarer's Course (for adult beginners), kicking our way through an ankle-deep carpet of ochre beech leaves, our maps and compasses in hand.
"Orienteering" is an easy sport to get into. You don't need any special equipment; you can just turn up at an event in your tracksuit and get going. "It's a great day out in the fresh air," Mr Robson said as we jogged past the scattered debris from a once great oak that was recently struck by lightning.
The programme for the November Classic, however, had made the event and the sport in general look fiendishly complex. The four-page leaflet that fell through my letterbox the day before was full of military-style rules, procedures, and details of the numerous different age classes from 18 to 75. It even had a section entitled Protests with instructions on how competitors can complain if they think they have been treated unfairly.
"We recognise this is a problem," he said. "The programme does look complicated at first because there are so many code letters and numbers and regulations that experienced orienteers would know but beginners would not, but when people come to an event they find it is far more straightforward.
"You can do what you like. If you want to compete there's a race where you might be up against world championship orienteers, or if you just want to take it easy and have some fun then you can do a less demanding course."
This makes orienteering a great leveller. All you need is to be physically capable of following the course on foot and at least familiar with a map and compass, although those skills can be explained by an official.
The idea of orienteering is to follow a route, marked on a map, with the aid of a compass to various "controls" - white-and-red, triangular banners. By each control hangs a hole punch which the orienteer must use to mark his or her route card. As each punch is different they verify that the controls were visited.
If your Saturday night was too boisterous to allow you to run far on Sunday afternoon, then simply walk the course. For children there is a String Course where young participants learn the basics of finding their way in the woods by following a trail of string around a short trail.
"People enjoy it because it is a good mix of physical and mental agility. You have to think out here, you have to observe the landscape around you to work out where you are and where you need to go next," said Mr Robson.
The officials were packing up as we stumbled up the last slope to the finish line. I was a little out of breath and my shaky right knee had started to nag but we had found our controls and rediscovered our way after losing it when crossing a mystery stream that was not marked on the map.
We were met at the finish by Tim Pugh, the director of the World Championship Committee. Britain is quite good at orienteering - we rank about fifth in the world - and our national team is currently training hard for the World Championships in Inverness next summer. Mr Pugh believes Britain has a real chance of winning.
"The Scandinavians are always very strong but we have some good orienteers at the moment, especially women. Yvette Hague is Britain's top woman having won silver and bronze medals in previous world championships," he said.
"We are hoping in a few years it will be accepted as an Olympic sport but for that you need 75 countries to be actively supporting it and so far there are only 50."
At next year's World Championships an amateur event will be held where the public can orienteer every day for a week between watching the professional races.
Orienteering may not be high-octane adventure but it can be a great day out in the countryside, either in tame and sheltered areas like the New Forest or more challenging environments like the Lake District or the Scottish Highlands. All cavemen and women welcome.
Start point: Contact the British Orienteering Federation on 01629 734042 for information on events and local clubs.Reuse content