I've got an 8 week old and I've had this problem 2x so far. It didn't turn to mastitis either time, so it doesn't have to with you either if you treat it agressively. My lactation consultant says they don't always go away quickly: my first lasted 6 hours, my second over 40. Also, even after it clears your breast can feel bruised/sore for a couple of more days.
My lactation consultant told me not to panic because plugged ducts are common and as long as I don't get any flu-like symptoms and treat the problem aggressively, I don't have an infection and will be ok. She advised me to call my midwife immediately if I get any flu-like symptoms, so I could get antibiotics. You should do the same.
All the advice you received thusfar is great. Try all those things. My lactation consultant says you only have to do the nursing every 2 hours for the first 24 hours or so to get the clearing started. After that, go back to your regular nursing schedule (as the baby allows--he needs to readjust too.) That was a relief to me after very little sleep! Her point was that if I keep nursing every 2 hours over several days, I'll cause oversupply which could make the problem worse.
Additional advice I have for you is to rest and stop doing other activities except eating, sleeping and nursing if at all possible (easier said than done unless you have someone to come wait on you and babysit your other child). Also, take ibuprofen as an anti-inflammatory.
Go to kellymom.com and search "plugged duct." They have an excellent chart of symptoms and treatments for plugged ducts and mastitis that really helped me. Feel free to email me directly if you have any questions. It's a painful problem and I hope yours clears very soon.