Without getting into too much detail, I am facing a dilemma as to how to address my Achilles tendonitis.

This came up after a twisted an ankle in November during Javelina and I continued to run for another 50km on the ankle. I finished 100km and then bowed out -- very sore Achilles soon followed.

I believe running with limited ankle mobility caused this to rear its ugly head. I have not had Achilles issues prior, and the MRI was negative that I had in December. Anyway, it is still bothering me and actually I tweaked it stand-up paddle boarding of all things a few weeks ago.

I already bailed on two races this year and have had low mileage, and now am facing the prospect of missing the two replacement races in March/April.

I would love to run: 1. Any distance at Zion in April, 2. Leadville in August and maybe 3. Diagonale des Fous in October. I am tempted to shut it down for 3-4 weeks, cycle only, and run maybe Zion 50km April 10 just for fund and see how it goes (I have a decent aerobic base now). I see so many conflicting ideas about resting (running) for a month versus running through and just icing, strengthening etc. I do not want to jeopardize my season and enjoyment in running, either by pushing it or by just doing no training where a layoff is not going to help anyway.

Ideas, experiences?

I can't say that I've ever "shut it down" in terms of running for more than 3-4 weeks. Sounds like you need to do something though. Having races on the schedule that you are not able to prepare for is added pressure. Seems to me that you need a few months of healthy running before considering summer and fall races.

I've been in the boat a few times where I had an upcoming race and soft tissue injuries leading up to the event. I just muddled through the remaining training, took extra cutback, extreme taper weeks, went out and completed these races (Boston 2005 comes to mind the most) and then banked a few weeks off afterwards. So maybe that sort of strategy will work for your April race but it does not sound like your training has spot on the last few months. In my example above I had relatively no issues until 6-7 weeks before race day. Maybe you need revaluate your spring season in terms of having a better summer and fall this year.

I tend to have problems with tedonitis even if I haven't injured a joint.  Last August I rolled my right ankle. It was at least a grade 2, maybe approaching a grade 3 sprain. I probably should have stayed off of it for a week but I didn't, hence healing was probably slower then it could have been.  I've been running on it all winter and working some at strengthing it. I still have residual soreness and stiffness in the ankle.  In January I developed soreness in the Achilles that coincided with a increase in mileage.

Several years ago I got my Dr to give me some Voltaren Gel. It works quite well for tendonitis, at least for me.  When I had my flare up I started using it, along with stretching more and I can tell a difference.  For some reason I find that the bike is really not much easier  on my achilles.

I won't offer an opinion on shutting it down, but I would recommend trying the Voltaren Gel. Since it's topical you don't have to worry about long term use and liver problems.

I am just starting running this week after two months of shutting it down for achilles issues.  I'm not sure of the cause of my tendinitis, but it worsened over the second half of 2014, probably from a combination of increased mileage, increased climbing and some form defects/ muscle weakness.  (I had never had achilles issues before last year.)  I took a few weeks off after running R2R2R in October, started back up again, and the achilles continued to hurt. For me, the pain wasn't bad when running, but pretty much every other time I felt like I was in pain and hobbling around - getting out of bed in the morning, going up and down stairs, etc.

So in December I just decided to shut it down for two months.  I cycled a lot, often at fairly high intensity, and after four weeks started doing the heel drop exercises that seem to be the standard protocol.  I just started running again on Saturday, easing into it slowly (3-4 miles, then hopping to the bike for the rest of the workout) - so far, no achilles pain, and my legs and lungs seem to be in fairly good shape following two months of cycling.

Based on my research and my experience (both somewhat limited), I would shut it down for a month or more - achilles tendinitis doesn't seem like the kind of injury that you can just run through.  Cross-train a lot (which should be feasible since you're a serious cyclist) and your training won't take that big of a hit.