In the 2-4 age group, I've found, with both my own kids and my grandkids, that the kid will want to walk a while on his/her own but then will tire out and need to be carried. But as soon as (s)he has rested a bit, there will be screams to get down again. The same is true of a stroller, as you may have noticed.

You have at least 1 1/2 years before the kid can complete a hike of 2 miles without being carried part-time. However, each child is an individual. Some may be totally independent by 3; others may need to be carried part-time until nearer 4. It does help if you have them walk as much as possible, because they'll get stronger sooner!

I'd get the carrier; you definitely need it now and for at least the next year, probably a bit longer. If it's a good one you can sell it once the child outgrows it. Our carrier (one of the earliest Gerry carriers) went through 4 kids. We had it 12 years and then sold it (thanks to inflation) for almost as much as my parents paid for it in the first place. (If I'd kept it, it would probably be worth a lot to anyone who collects "antique" hiking gear!) Last spring, my youngest son sold the Kelty carrier I bought for their first child; their youngest child was 4 in October. It was more a case of the kid's getting too heavy. The problem with this particular child is to modify his "hiking" style, which consists of a dead run until he tires out.

If cost is an issue, look for a good used carrier.