Dont know that model of carrier you are using but can give you a bit of info from our hiking experiences...
We are pretty out-doorsy (dh VERY!). Our ds is now 22 months and quite happily sits in his back pack. However, he has screamed a fair bit over the last months that we have used it, and the truth is, there are numerous reasons for it.
You have to accept that your little person is going to have their own feelings about things and can't communicate it to you. It might be simply that they are not enjoying it, in which case it is up to you to decide whether is a day where they are the priority or a day where your walk is the priority!!
Biggest thing to look out for - cold hands or feet - or cold in general. Remember they are not moving up there, and need to be wrapped up. Feet esp. OUr son wept and refused to put his feet down when we reached the top of a hill when he was 18months. It was only then that I found out his feet were blocks of ice
I would say make sure your baby is kitted out as best you can - I am still looking for a good (cheap) solution to little feet being cosy - not quite ready to fork out for baby scapas yet.
My best advice would be to keep at it, so they get really used to it, but to do shorter walks, and not be too ambitious. We have found that it is better to do an hour's walk up a modest hill with a lovely view, and a happy baby having fun at the park afterwards than attempting hours of munroes. Also, 6m is still quite young, be patient and you will be able to extend the time as they get older
Hope some of that is useful (and not too depressing!)
Happy Hiking!