In reply to franny:
baby bjorn is fine facing inwards (on your front) for smaller babies - it's turned outward position when a bit bigger that is more "crotch dangling".
I love the ergo (for carrying on back from about 6 months) - even though baby had ace head control before then had to wait until legs would comfortably open into the required piggy back pose. On back is so much better for your back.
If you love running - try make it you time and get a break from the baby (leaning to push might be bad anyway if you have a bad back). Alternatively you could run laps round a parked buggy.
Think I would get an ergo carrier - with the newborn insert if I were buying from scratch.
I prefer the ergo (soft structured type of carrier over rucksack/ frame type) as much lighter, can be carried easily in bag or bottom of push chair, can be used for assisted piggy back up to about 4 years old (dependent on size of toddler/ child. Whith the ergo the weight of the baby is much closer to your body, not levered outwards (if you see what I mean).
When they are little and on the front wear a big (partners?) coat if its cold - then you know they are warm - not dangling on the outside in not very outdoorsy baby coats/ suits.
Even on back you can kind of put your coat over them - depending on what you're doing.