Allow me to add some substance to this claim!
Around the time I completed the Couch to 5k plan, a friend of mine ran a half marathon. I was in awe of this chap. Whilst I was struggling to get through 5k, he was running almost 5 time that distance!
I recall chatting about his achievement and saying I was thinking of entering myself into a 10k but needed time to build up from 5k to 10k and how long would that take? Weeks? Months? To which he stated that during his training for a half marathon, he never ran more than 5 miles, indeed he'd never run more than 5 miles in one go in his life, then turned up on the day of the race and ran 13 of them! He claimed "if you're a running 5k two or three times a week - you can run 10k"
I thought he was off his rocker! I'd run 5k about half a dozen times at this stage, never anything further. But one day I decide to give it a try, to see if he was right. And low and beyond, I ran just over an hour and ran for 10km's!!
I couldn't believe it. I'd only started the plan about 12 weeks previously and I'd just run for 10km's! I was absolutely over the moon!
I firmly believe that, within reason, what ever distance you set off to run, you will run. If you have the stamina to run for 5km's, you already have the stamina to run for 10.
On Monday last week, I decided to run home from work, its just under 13km. I'd only ever run 10km before and only done that twice. But I did it.
Three days later I wanted to squeeze in a run because I knew my plans wouldn't allow to time until the weekend so I set out to run 5km. I was exhausted at the end! I barely got to the "finish"! The thought of running another 8km like I'd done a few days previously seemed ridiculous, I was knackered!
The point of all this rambling is I personally believe that once you get to the stage where you can run for around 30 minutes none stop, you're fit enough to run further. It's all in the mind.
Set out to run 5km, you'll need to stop after 5km.
Set out to run 7.5km, you'll feel ok after 5km and be able to manage the extra 2.5km.
Set out to run 10km and I bet its the same, take it steady and pace yourself from the off and you'll run 10km!
For those asking about the progression from 5km to 10km - I bet you can already do it. Trust your Couch to 5k achievement, trust your fitness, trust your body and trust in your ability. I'm convinced you're already there!