RUN SERIES: Should you always try to run faster than last time?

Maybe it's just me, but whenever I head out to train I always want to push harder and work faster than the time before. The concept of an 'easy run' did not compute with me at all. I couldn't get my head around the thought that running slowly - or, at least, slower than my hopeful marathon pace - could be beneficial.

That was all before I met Tom Craggs. At our first London Marathon prep evening, courtesy of Lucozade Sport, I had the pleasure of a training session with one of the UK's most sought-after running coaches. It may have been a dark and chilly night, but we headed out along the South Bank for a threshold and interval running session. During our warm up jog, I got to quiz Tom on the question that had been bugging me since I started taking my running slightly more seriously: how does someone as determined as I am get to grips with the concept of easy running - and why should we?

"It's a matter of training efficiently and understanding the different energy systems your body will use to get you through an endurance event such as the London Marathon," says Tom.

"Even with all the carbs and gels in the world, your body is not going to get through 26.2 miles without recruiting your body fat as an energy source. You need to get your body used to doing this and not relying on your more readily-available glycogen stores in your training runs to get ready for Race Day."

So how do we go about doing this? Stick with me here through a little bit of physiology - I promise not to get too technical! Firstly, think about the two types of muscle fibres in the body: what they do and how they are fuelled.

Fast twitch - good for power, speed and short, explosive movements

Fast twitch muscles require an immediate energy source, so tend to use the ATP and anaerobic systems which work incredibly fast to provide the body with the energy it needs using stored ATP and glycogen (sugars). If I go to sprint 200m, or do 50 burpees (everyone's favourite) I will be forcing my body to use its stored carbohydrates to keep me going for the few minutes I'm working.

Slow twitch - better for endurance and lower-intensity exercises

On the other hand, slow twitch muscles essentially have the time to convert body fat into a fuel source, because they don't need the fuel as immediately as fast twitch muscles do. If I run for more than about 2 minutes, or do (god forbid) 20 minutes of burpees, I will have to drop the intensity slightly and will need to recruit my slow twitch muscles for endurance. This is where your body starts to use the aerobic system, and this is where, in addition to using glycogen, you can burn fat as a fuel.

intensity duration energy curve marathon running energy systems chart.jpg

In a marathon context, this is essential to getting round the course because our bodies cannot rely solely on stored glycogen (which runs out after about 90 minutes of exercise). Yes we can take on gels and Lucozade Sport to replenish glycogen (and important electrolyte) stores, but even these won't be sufficient on their own. By slowing down our long training runs, so we're not going flat out, we're actually training more efficiently and encouraging our bodies to burn our stored fat as a way to keep us going. It's also essential for sufficient recovery in between long runs. Craggs' advice is not to tackle a 26-miler before the marathon itself, or to run your longest training runs at marathon pace. Why? "Because it takes the body a long time to recover from that kind of energy output. You want to arrive on Race Day feeling fresh and ready to keep pushing past that 18-20-mile mark, not worn out from a hard run a few weeks before!".

It's certainly working for me and has really helped me to understand my pacing. I'm even starting to experiment with ketosis as a primary fuelling mechanism to improve my endurance...more on this soon!

You can find out more about Tom Craggs here.

Lucy Denver