In reply to Treck:
I live in the Netherlands too and don't train except for my daily bike commute and some casual gym climbing / bouldering. To give you an indication: When I go for a run I do 3 km in >20 minutes and feel knackered afterwards. In the Alps, I start with a week of shorter walks and climbs to get acclimatized to the mountain environment and let my legs get used to steep walking. This has thus far done the trick for me on steep 2+ hour approaches with climbing and bivy gear on my back. It has more to do with listening to your body, easing in to it, controlling your pace and taking a rest/recovery day every now and then.
Also:
Use trekking poles(!)
Remember to eat and drink at regular intervals and rehydrate well before leaving the hut
Slow your pace to a level that allows you to take fewer rests
Wear the minimal amount of clothing layers to reduce sweating and overheating
Have your rope-work and route-finding dialed and get fast due to efficiency instead of superhuman stamina.
Cut your pack weight to the bare minimum (very effective but requires some experience)
When staying in a mountain hut during your course: drink Radler beer instead of regular beer to aid recovery.
Post edited at 15:17