Hampstead Heath, perimeter 8.0mi (13k)
Lose yourselves in acre-upon-rolling-acre of woodlands and lakes. The views from Parliament Hill and the Pergola (West Heath) provide ideal picnic material, if only it wasn't February and so flipping freezing!
Pros: Great views and great for romantic explorations.
Cons: Some trails will be muddy. He or she may not like that, so be careful how you frame the comment "let's get dirty today".
Home Park (Hampton Court Park), perimeter 5.0mi (8k)
The quieter of the Royal Parks, this is the ideal spot for some deep-and-meaningfuls while jogging through the gently waving grasses, past the glistening Long Water as herds of grazing wild deer watch you run by.
Pros: Solitude (relatively speaking) and opportunity for a sweaty snog in front of a palace.
Cons: Your date may not be into sweaty snogs (pre-run snogs are usually your best bet).
High Elms Country Park, perimeter 2.3mi (3.6k)
A nature reserve amongst sublime Bromley countryside. Start with a lap of the High Elms Trail and then run south to Charles Darwin's House where you can elaborate on the evolution of your love for each other (better not mention anything about survival of the fittest though).
Pros: Feels a world away from the hustle and bustle of the city.
Cons: Is a world a way from the city by the time you've caught the train to Chelsfield and then jogged 2.0mi to the park (might want to take the car to this one!).
Valentines Park, perimeter 2.0mi (3.2k)
This east London park (in Ilford) has 50ha of formal parkland, a boating lake, plenty of ducks to feed, a palace, and two lovely cafes to grab brunch. Plus Wanstead Park is just 0.6mi to the west, meaning you can take your date on a big adventure through the southern reaches of Epping Forest and finish at this cute spot.
Pros: Everything you'd expect from a park bearing this name.
Cons: Everyone else in Redbridge will probably bring their Valentine here as well.
Greenwich Park, perimeter 2.0mi (3.2k)
Nothing says "I love you" like running to the end of the world for the love of your life. Here you can technically do just that by running to our planet's longitudinal zero. Also add-on a trip under the Thames via the foot tunnel near the Cutty Sark. And if you're all starry-eyed, why not top off your run with a visit to the observatory?
Pros: Greenwich has that royal air about it that shows how much you care for your prince or princess.
Cons: Don't try to beat your Valentine to the top of the hills—this is not the time to impress them with your incredibly advanced VO2max … save that for later… [cough].