sanpokoosu ha nagai you da.

Here, ようだ means something like it seems, it appears, or it looks like.

So 散歩コースは長いようだ means the speaker is making an inference, not stating a hard fact with full certainty.

It has a softer nuance than just saying:

• 散歩コースは長い。 = The walking route is long.

With ようだ, it becomes more like:

• The walking route seems long.
• It looks like the walking route is long.

This is often used when the speaker is judging from some evidence, impression, or available information.

In ようだ, the だ is part of the grammar pattern.

You can think of よう as a noun-like word, and だ is the plain form of the copula attached to it. So ようだ works as a set expression meaning seems to be or appears to be.

Even though the sentence already has the adjective 長い, you still use だ because it belongs to ようだ, not to 長い.

Compare:

• 長い = is long
• 長いようだ = seems to be long

In polite style, this becomes:

• 散歩コースは長いようです。

は marks 散歩コース as the topic of the sentence.

So the sentence is talking about the walking route, and then says something about it:

• As for the walking route, it seems long.

This does not necessarily mean strong contrast here. It is just the normal topic marker.

If you used が instead, the nuance would shift slightly depending on context, often sounding more like identifying or emphasizing that subject. But は is very natural when introducing what you are talking about.

散歩コース is a compound noun:

• 散歩 = walk, զբոսանք, stroll
• コース = course, route, path

Together, it means something like:

• walking route
• route for a walk
• walking course

In everyday Japanese, compounds like this are very common. Even though コース comes from English, in Japanese it often means a route, track, planned path, or set course.

Because ようだ attaches to the plain form of the word before it.

For an i-adjective, that means using the dictionary/plain form:

• 長い → 長いようだ

Not:

• 長くようだ ❌

長く is the adverbial form of 長い, and that is not what this grammar pattern needs.

A few patterns:

• Verb: 行くようだ
• i-adjective: 長いようだ
• na-adjective: 静かなようだ
• Noun: 学生のようだ

This is a very common question, because both can be translated as looks/seems long.

A simple way to understand the difference:

• 長そうだ: looks long at first glance; based on appearance or immediate impression
• 長いようだ: seems to be long; a broader inference based on evidence, information, or judgment

Examples:

• この道は長そうだ。
  This road looks long.
  Maybe you can see it stretching far away.

• 地図を見ると、この散歩コースは長いようだ。
  Looking at the map, this walking route seems long.
  This is more of a conclusion from information.

So in your sentence, ようだ sounds a bit more thoughtful or inferential.

Not necessarily unsure, but definitely less direct and less absolute than a plain statement.

Compare:

• 散歩コースは長い。
  The speaker states it as a fact.

• 散歩コースは長いようだ。
  The speaker is saying that, based on what they know, that seems to be the case.

It can imply:

• the speaker has not fully confirmed it personally
• the speaker is judging from clues
• the speaker wants to sound less blunt

So it is often about inference rather than simple uncertainty.

Yes. ようだ is very flexible.

Here are the main patterns:

• Verb + ようだ
  雨が降るようだ。
  It seems it will rain.

• i-adjective + ようだ
  この道は長いようだ。
  This road seems long.

• na-adjective + な + ようだ
  ここは静かなようだ。
  This place seems quiet.

• Noun + の + ようだ
  彼は先生のようだ。
  He seems to be a teacher.

So with 長い, the sentence is using the i-adjective pattern.

Yes, it is grammatically complete and natural.

That said, in real conversation, Japanese often includes some context showing why the speaker thinks so. For example:

• 地図で見ると、散歩コースは長いようだ。
  Judging from the map, the walking route seems long.

Without extra context, the sentence still works fine. It just sounds like the speaker has some basis for the judgment that is either obvious from context or left unstated.

The original sentence is in plain style:

• 散歩コースは長いようだ。

Polite version:

• 散歩コースは長いようです。

More casual/conversational alternatives:

• 散歩コースは長いみたい。
• 散歩コース、長そう。
  This one changes the nuance a bit, closer to looks long.

So ようだ is natural, but in everyday casual speech, many speakers often use みたい instead.