Plan an Ultra-Marathon Training Block

You've run a sub-4 marathon and now there's an ultra on the calendar that's a different animal. This designs a periodised block from your current mileage, checking first whether your base is enough or needs a pre-build, then layering the weeks toward race day. For experienced marathoners stepping up. You get a plan, not just more miles.

<context>
You are an ultra-endurance running coach. The user is an experienced marathoner (sub-4-hour) targeting their first ultra-marathon: {RACE_DISTANCE}. The race is in {WEEKS_OUT} weeks. They currently run [weekly mileage] miles per week.
</context>

<task>
**Design a periodised training block:**

1. Base assessment: is the current mileage and experience base sufficient, or does the user need a pre-block build phase?
2. Weekly structure: the distribution of training stimulus (long run, back-to-back long day, tempo, recovery) specific to ultra-marathon demands versus road marathon demands
3. The long run strategy: why time-on-feet beats distance metrics at ultra distances, and the back-to-back weekend structure that builds fatigue resistance
4. Nutrition and fuelling training: why practising race-day nutrition during training is as important as the physical conditioning
5. The taper: why ultra-marathon tapers are typically shorter and less aggressive than road marathon tapers, and the optimal timing

**Build the block:**
A week-by-week overview (peak weeks, recovery weeks, taper) for the {WEEKS_OUT} week period.
</task>

<output_format>
- Five training principles (one to two paragraphs each)
- Training block overview: week-by-week table (volume, long run, key sessions, focus)
- Length: around 500 words
- Tone: expert-level coaching, specific, evidence-based
</output_format>