Mutrie suggests a simple walking programme. "Aim to increase the amount you are walking over the course of a month. Start with walking for 10 minutes a day and gradually up it." Most smartphones now have pedometers, so you can track your steps: 7,000 steps a day is a good target to set yourself, with the aim of eventually increasing it to the government recommendation of 10,000 steps daily.
Jermaine Johnson, another personal trainer who often trains clients with depression and anxiety, suggests avoiding cardio. "That can raise the heart rate and fuel your anxiety," he says. Instead, he suggests that you try strength training. "I've trained people with depression who've said weightlifting made a big difference." Because you can measure your progress with weight training - heavier weights or more repetitions - you will be able to see how much you are improving, which could lift your mood.
You don't have to exercise all the time. "If you want to optimise your exercise regime for your mental health, according to our research, exercising for 45 minutes at a time, three to five times a week, has the most beneficial mental health effects," says Dr Adam Chekroud of Yale School of Medicine. He is the co-author of one of the biggest studies into exercise and mental health, which looked at 1.2 million adults in the US. He says that cycling and team sports will give you the biggest boost, but even walking or doing household chores are better than nothing.
It's a good idea to pick times when gyms are less busy, so you won't feel like you are being watched. "Speak to the gym manager and ask them about their quiet times and quiet areas," Overall says. If you're able to slip away from work, gyms are generally quiet in the mid-afternoon. Plus, you won't have to drag yourself out of bed at the crack of dawn. "If it's dark or rainy, it can be hard to get up early, especially if you're struggling with depression," she says.
Just being in nature has also been shown to have beneficial mental health effects. "You need sunlight for vitamin D, and vitamin D lifts your mood," says Overall. Look for a Green Gym near you: these free, conservation-based workouts are a great way to meet people in your local area, if you're feeling lonely; sign up and you could be planting trees or sowing seeds alongside other participants. Beyond that, the mental health charity Mind has a directory of fitness classes and other outdoor activities on its website (mind.org.uk), many of which are free. If you feel up to being around other people, group exercise can help. "When it comes to depression or stress, these conditions have a biological underpinning, but they also have social components," Chekroud says. The sense of community you feel as part of a sports team can have a brightening effect on your mood. "It takes the attention away from the individual person," says Johnson. And if you end up feeling overwhelmed or anxious in the class, it is fine to leave early. "You always have the option to leave. I've had people who've walked out of my classes," he says. "That's fine." If you're feeling very anxious before a class, give the trainer a heads-up that you're not feeling great, and might slip out early - and pick a spot near the door for minimum fuss.