Hi Dorota (and all of you wonderful moms!):
You posted a good and important question, how do I know if the goal I set myself is too lofty? The answer will be different for each of you. The most important question to ask yourself, especially if you are newly postpartum is "Does exercise energize or deplete me"? The end result of your training should be rejuvination (maybe not immediately...but in the long run) To hit the Half Marathon mark by May, you would need to be ramping up in Feb to 2-3 days of 4-5 miles and your long days from 6-7, then in March 2-3 days of 6-7 and your long runs at 8-9. By April, your runs on those 2-3 days should consistently be around 8-9 miles with your long runs between 10-12. Depending on your race day, you would tamper that mileage off a little at a time the last week to a week and a half leading up to race day.
Ask yourself if this seems do-able...maybe start February out with this plan and see how you feel. Again, if 2 hours after you've ran/trained you realized that you are completely wiped out, you went too hard/too soon. This decision to change your end goal doesn't have to be made right now, start the process and have a back up race in mind. Training as a mom is a lot like motherhood in general: Go for your goals with a plan but know that sometimes we have to "punt" and that's ok!!!