Question about increasing speed with the use of Heart Rate Monitor

I'm training for a half-marathon in February, why shouldn't I be doing intervals? And what's this "off season" thing?

Won't be as fast as what? My big fat @$$ was last year?

Glad to be of service ma'am!

Concentrate yes, but it's more of a benefit to focus on building a stronger base and working on technique now so that you can take advantage of interval and speed work as you build and peak for your A races during the season.  Intervals and speed work are the final stages of race prep and you can typically only hold that peak for a couple weeks before the speed starts to erode...which is why periodicity is used through the season.  Unfortunately the majority of the "speed" you build now will be gone by the spring and you'll have to earn it again for your summer races.  Like Ken said, if you have a winter race that you consider an A race then go ahead and do your interval training but otherwise it might be best to leave the Z3+ stuff out of your training plan for a few months. 

Which brings me to another point for the original poster...have you estimated your LT and determined your training zones?  If not I might suggest doing this ASAP so that you can set upa more effective training plan.  I don't know where 160bpm puts you but for me that is just into Z3...a bit too intense for this point in my training cycle in most cases.  I prefer to hold in Z1/Z2 for 2-3 months.   Also, as someone else suggested, this might be a problem of pacing during the swim/bike portion and/or lack of brick workouts to prepare for the bike/run transition.

One of my more common suggestions is to sign up for a silver/gold membership and take advantage of Mike Ricci in the training support forum.  He'll walk you through establishing your training zones and provide support for any questions you might have as you work through any of the training plans available here at BT.  It's really a great opportinuty to work with an experienced coach and get direct personal feedback without the expense of a personal coach.  I see you are new around here so take some time to look around and see if you think this site suits your personality and needs.  It's growing (features and members) constantly and the memberships are a good way to support the site as well as get access to additional features.